As my Mom's POA, how can I stop my brother from overspending her money if she willingly gives it to him?

Yes, you, as POA, should conduct it as a business. If your brother is not happy with it he should not be a caregiver. Schedule monthly meetings with all involved in the process where you discuss all your mom's needs, how much her care cost and how it will be approached. Her atty could be on the phone for that meeting too.
Investigate how much living-in-caregiver would cost in your area. Set up limits for how much your brother can "charge" for caring based on what the current market is. Yes, it is fare for your brother to be paid too if he is there 24/7. And tell him how much you appreciate all his hard work. But explain that you are accountable and have to make sure your mom has enough money through the years to come.
Write it down! Document it: protect yourself.
I saw families falling apart because they could not "remember" what was said before, who is responsible, etc... They point fingers to each other and can't prove anything. Family courts are flooded with families disputing their stories.
Learn from other's mistakes, not your own. And protect your mother's interest. At the end, if your brother is abusing your mother's funds, current situation may cost you much more than to hire caregivers. And, the "worse" part of it, YOU are responsible for that future now as POA!

By the time my friend realized that her brother who was supposed to have been "managing her father's money it was nearly all gone! Her sister in law charged her father an extra $500 as payment for doing his grocery shopping, wrote checks to her kids for presents for $100's, and paid for her children's college tuition for college! Her father's house was in deplorable condition, he was dirty, un-shaving, and needed care. Since my friend lived in another state she had no idea that this was going on at this level. She ended up rescuing her father and bringing him to live with her. Only under threats to go to the courts did her brother repay some of the money so that she had means to care for him. Unless you do something soon you may be in the same trouble as my friend was having to care for your mother at your own expense. If you cut off your brother's money he may retaliate by forcing you to care for her or at least arrange for her care. You need to get to a lawyer quickly. Good luck, fighting about money in families is one of the ways families get torn apart. Your mother's best interest should come first!

Is he taking care of her while living there? Do you know if any of this money is going towards her needs? Yes, his history isn't the best. But before you assume all of it is for him alone, you might want to find out answers to those and some other questions. This way when you see lawyer or bank or whoever, it will show you weren't accusing him but trying to truly understand. If Mom has just a little short term memory loss and is aware most of the time, then she is freely giving this money. Perhaps another way to manage it is stop the ATM card and give Mom cash weekly that she can use as she wishes. After all it is her money. If you can't stop the ATM card then set up another account for you and Mom, move the money into that, and set up weekly transfers to the account the ATM card is associated with. No money in the account when he goes there? Then withdrawal will be denied.

Addendum- Also, if you don't mind some advice..like one of the other posters was saying, get him a diagnosis. That really helped in my dad's situation.

Hello there! I have the POA for my dad. He also has memory loss, not dementia but Mild Cognitive Impairment. If I were you, I would follow the other posters advice of closing the account. IMO, it might come back on you on why you allowed this to happen. (I don't like saying that)

I am so sorry to hear that your brother is doing this to your mother. If your name is on the account with your mom he shouldn't be getting away with this at the bank. If the account is set up with your moms and his name he might have the freedom to do this with no questions ask. What I would do is go to the bank and find out how you can close the account and start up a new one. Your mom would have to sign. Show the bank documentation of POA. Are you also durable power of attorney for her health care. Does your mom get direct deposit or automatic withdraws from this account ? That would all need to be changed when you close the account. Your bother needs to stop manipulating his mom. What a shame! My brothers did the same to my mom. She gave them money freely. The would drive her to the bank so she can give the extra money and didn't have money for herself. She lived in Florida at the time and finally I convinced her to move in my neighborhood. It didn't stop. Finally. It did because I had said to her she can do it only when her needs are met. Have her statement paperless online so you will be the only one to see it. I hope these suggestions are helpful. Be patient and with gods help you can do good for your mom.

one more thing..... or maybe several things as they come to me today....;)
Even if THE LAW tells you what you can and can not do ....... dig deep in your heart and soul to see if you have any "funny feelings" about what THE LAW is saying is ok. Think back when you two were siblings and things went on in the family. Try not to stand behind THE LAW with all the decisions you choose to make. Sometimes THE LAW is so unfeeling and insensitive that THE LAW could CARE LESS that two people on the earth (you and your brother) will NEVER speak together again after your mom passes onto a MUCH better place.... or onto H-E....double hockey sticks. whichever the case might be.....;)
Since all this is heading into a crisis and heads up THIS WEEKEND for me.... I know I will have a lot to say on this subject as today goes on. I apologize NOW if I hog this page or whatever you call it when someone types a lot when people have NO INTEREST in what is being typed!!!
kathy from CT

You have POA, right? You close her current account and open a new one. You set it up so that only you can make withdrawals. If the bank won't do this, talk to and Eldercare attorney or go to your local Legal Aid. Get your mom to a neurologist for a diagnosis.

I just thought of something.... get an attorney, a financial advisor, your brother, your mom and a social worker all in one room together. (This might be a tad difficult. In from of all the experts... you and your brother plead your cases and have the attorney, the financial advisor and the social worker decide on how much your brother should be able to withdraw each week. If you mom has dementia and can't put in a clear vote.... and she definitely has NO IDEA what the meeting is about........ have a relative and/or good friend sit in on the meeing with her and share what your mom's views have been your brother's whole life about her son. This may bring some clarity to you about your brother and mother that you never knows about them and THEIR relationship and how he feels "entitled" to mom's ATM access....

Please people on this site.......give some feedback even if you feel it is "stupid" or not related to this question. for some reason everything seems to be related in some way......

Why does he have an ATM card to her account? I would go to the bank and shut the card down. It would probably be a good idea to see a lawyer about this also.

I have NO IDEA!!!!! I am experienceing the EXACT same thing in my situation with some of the details a little different!!!

Please please please! someone out there give us answers on what to do!!!