I'm going to refuse to support my drug addict brother and father.
What do you mean by refuse to support them?
I'm not going to buy them any food, and I'm not going to loan them any money.
Although what you are doing is difficult and they will almost certainly be angry at you for it, you are doing the right thing. Cutting them off is better than fueling their addiction. You should try to get them addiction therapy if you can.
I'm tired of being pushed around and constantly giving them money.
I don't blame you for being frustrated with the situation. I think you are doing the right thing by cutting them off, and they will eventually hopefully understand why you did it. If they get past their addiction, I'm sure they will thank you for forcing them to come to grips with their situation.