Fired caregiver still visits mom at her house. Is there anything I can do to protect my mom?

Is anyone the PoA for you mom? If so, this person should read the PoA doc to see when the authority is activated. If this person is able to activate their authority then the PoA can get a restraining order. If no one is the PoA and she doesn't have a diagnosis of incapacity, there is nothing to be done except contacting APS to report her if the alcoholism causes her to need help.

Do you live locally to her?

"I don't care if this caregiver comes over and provides company to my mom but buying wine for my mom and serving it to her..." She is not there to keep her company but to take advantage of her. She knows your mom's care plan and doesn't care.

Well, this comes down to what power you actually have. Your mother, it sounds like, is living independently at this time. I am assuming that means that she has no diagnosis of dementia, that you are not her designated POA with implementation of those duties. If I am right, there quite honestly nothing you can do. Especially if the agency is already taking action against this person. If this person is now saying "Well, you fired me. Fine. But this is still my friend and we have cocktails at 5:00" I suspect there is little you can do.
Your Mom apparently, according to your assessment, has a drinking problem. You can't control that. I would attend Al-Anon just to let you get how helpless you actually are. Until Mom goes down, breaking a hip or some other bones, there is likely little you can do; even then, when all is said and done, and rehab is over, there is nothing to change things your Mom doesn't wish changed.
I am so sorry you are going through this. But your Mom is either competent or she is not. If she is not she should not be alone living independently. That would give some power to someone.
So sorry you are going through this. I wish you the best. And your Mom as well.

change the locks

Does mom abuse alcohol? Or does she enjoy a glass of wine now and then?

Call the local authorities since this is a case of endangering a senior.

I assume your mom is letting her in. Charges need to be filed either through the Home Health Company or by you.

If mom has been diagnosed with dementia you should be able to file a restraining order especially if you are power of attorney.

If she is healthy,lucid and wants her company there is nothing you can do. I would install Nest cameras to monitor activities when mom is alone. We installed for elderly aunt to monitor her care and possible falls.

Someone mentioned changing the locks but this will not help if momma is letting her in.

Call Adult Protective Services?

Just read your profile indicating your mother is memory impaired.  This might be the key to getting assistance for your mother against the former caregiver.

aj6044: The locks should be changed on your mother's apartment. Visits are unwelcome from this fired ex caregiver.

First of all, there's no reason on earth why the care agency your mother uses should know who her friends are or who visits her apartment. It's none of their damn business.
No, there really isn't anything you can do except talk to her former caregiver/friend who visits and ask (not tell) her to stop bringing wine for your mom. Let her know that you appreciate that she was more than just a hired caregiver to you mom. That she became a friend and stayed one. Then ultimately it will be up to your mother. It's her choice whether she wants to drink or not. Honestly though from what you say in your profile about how bad off your mother is from a stroke, does it really matter if she has a few drinks at this point?
Maybe ask her friend to not let her get loaded on the wine. Maybe she would be willing to water it down for your mom.

This situation is totally unprofessional and inappropriate on the caregiver. Call the agency and report this. No one should be buying and serving alcohol to an elderly “ patient/ former patient, especially a so called “ care giver”. Good luck