Anathaine, yes, you need to cut "John" off. He is dangerous to you; he is supplying you with CS and using you for sex.
Since you feel worried that you will contact him, could you explain to him that you need to get clean and that, even if you ask, he must not supply you with drugs? From what I have read, I'm doubtful he would respect this decision - he gains from your addiction - but I could be wrong.
Be strong. Best wishes to you.