re: I would have zero problem paying an exterminator in this case...

The solution has already been figured out. The new owners have the deed to the house. They simply find a friendly "participant" to legally lease the home from them. This participant then, with lease in hand, goes to the home, walks in (breaking the door if necessary-with owner's permission). Once inside, he calls the police about the "burglar" inside. Police come, sees a valid lease for the participant and none for the squatter. Squatter is told to leave or be arrested for trespassing. Once squatter leaves, house is turned over to owners.

This has been used successfully many times, even in commiefornia.