Dad has dementia. He gets angry when one tells him no he can't buy something he shouldn't have or doesn't need. How do we deal with this?

He feels his ability to make decisons slipping away and doesn't like it that others see this. It's fear, I'd think. Has he seen a doctor? He likely needs treatment. There are medications that may help his anger and anxiety to some degree. I'd get him to a doctor. Also, contact your local Alzheimer's organization for suggestions.
Carol

Maybe let him buy whatever it is and then take it back to the store. It is extra work / running around for you, but at least it prevents the hitting thing - obviously just a temporary fix. I do this with my FIL..... The other mechanism we use is just not taking him to the store - we ask for a "list" and then conveniently forget whatever it is he is asking for that we already have.... it is ridiculous sometimes.

If you were to put him on a budget, giving him x amount of money in his wallet weekly, would he stick to just spending that or does he want more money? Would he understand that once the money in his wallet is gone, the spending light is out?

I do the same thing as ann1 does. I finally realized that arguing with her didn't work so I now tell my mother to make a list and I will get her whatever she needs.....She never does make the list ..

I do the same thing as ann1 also. I ask my Mom and Dad to make a list of whatever they need. Usually they forget to make the list and if they do I just don't mention the things I didn't buy from the list and they usually forget they asked for them. It might work to give your Dad an "allowance" to purchase things he wants and when it's gone, it's gone. As long as things are affordable I allow them to eat and drink what they like. My doctor said at their age anything in moderation is fine.