Hire a couple of actors to play the part of bikers and “rent” them a room. Pour human feces on the floors. (A repossessed house I bought in a really nice neighborhood had human feces in two rooms. I did get a great deal, though. Fortunately, it was winter when I donned PPE.)
I read a story not to long ago about a 65 year old man having the same issue. A couple from New York moved into his loft and then refused to leave. He spent months trying to get them out to no avail. His solution was to take 3 Black Widow spiders and slip them under the door. One of the renters was bit in the toe which ended up being very painful. There was no proof the old man had anything to do with it as the spiders are native to the area. The couple moved out as soon as they got back from the hospital.