Well, don't they leave the house or apartment at some point? Maybe to go out for coffee or food? I would just switch the locks and then gather their stuff up and put it at the curb. Of course, I would video this part, as well as me asking them to leave and them refusing.
Lacking that, I would call in my big, Amazonian friends to escort them out and then change the locks.
It would be truly awful, if a guest stayed more than 30 days and refused to leave, because at that point, tenant rights take over and there is not much to do, except get a legal eviction, which can take months to do.