I'm not going to give a homeless man money
Why aren't you going to donate him some money?
Because he'll spend it on alcohol.
I understand your concerns, and you are right to not need to give money to anyone if you don't want to. You really don't know if you're only enabling that stranger's bad behavior.
I know the guy, he is an alcoholic and probably going to drink the money I give him.
It seems like you have a good reason to not give him money. It is also okay not to want to donate money at all if you don't want to. It is not an obligation.