I don't want my son seeing his mother?

Well my son is 14 and I am his legal guardian. Me and his mother divorced when he was 5 and I got custody because his mother was an alcoholic and didn't work. I didn't mind my son seeing his mother when he got back in contact with her 3 years ago. But I found out she is giving him cigarettes and alcohol whenever he goes there and quite personally I don't want him to turn into a dirty whore like she did. I tell him not to go round her house but he ignores me and gets the bus there every weekend. It started getting out of control so I grounded him for 1 week every time I caught him but 1 week lead to 2 weeks and 2 weeks lead to 3 weeks and I couldn't punish him anymore because he became defiant and it is all that cheating whores fault who ruined our marriage. He has ran away from home and he has been living round the whores house for a week. I'm tired of this ugly whore corrupting my son. What should I do? I have run out of options.