I don't know if this makes any sense to you, or if you're even capable of doing it -- but a friend of mine whose son spent some time in prison on drug charges decided to just not accept his calls anymore. She wrote him a letter explaining that she would love to talk to him, but she was not sending him any more money, and if he brought up the issue of money, she would hang up on him.
That was her boundary -- and it did make her son relent about the money and eventually just call to have a conversation.