How to set boundaries when your SO won't
My 18yo SS lives with us full time, has basically dropped out of college, doesn't work, and does nothing but sleep all day and party/play Xbox all night. He'll sneak friends into our house when we're asleep, and I'll come down in the morning to find a stranger on my couch. I've told DH that I don't feel safe in this environment, and he basically tells me that I should look at it like we're helping keep these kids safe by not making them drive home drunk (or some such convoluted crap). He basically thinks his kid is not doing anything wrong, as long as he's not drinking/smoking pot in the house. I find cigarette butts/roaches in our driveway, though, and when DH points it out to SS he just walks past them and ignores him. (All his kids are like this, we just only have one of them living with us at the moment.)
Since DH doesn't enforce anything, SS does whatever he wants and basically ignores me when I tell him if I don't like a certain behavior, etc. So my question is, short of calling the police when there are people in my house or flat-out leaving this f-ed up dysfunctional family, are any other boundaries I can set? I already don't do anything for SS and don't give him money, etc. I basically try to avoid him whenever possible. But that's neither here nor there, since it's DH who's the problem. What are some successful boundaries people have set that have gotten SO's to come around? Do I stop paying for half of the bills and say I'm only going to pay for a third of them, since I think SS should be held accountable for something? Do I withhold sex until DH puts his foot down with SS? (Kidding. Kind of.) But seriously, what are some good strategies that you've found get the point across without being manipulative?
Or, more realistically, how did you come to terms with the fact that your SO will never change?