Hi Dan
First thing I would do is call the police and while you wait for them, go on your local Gov website to see what your options are.
Last year I had a request from 4 woman from Holland who wanted to rent my place in St Maarten for SIX month. Only problem, they could only pay for the first month upfront and would pay subsequent months from their pay checks as they planned to be workng on island during their stay. This went back and forth trying to work out a payment schdule for a few weeks until I got fedup, because none of mys uggestions for payment worked for them, so I said they should just look for another place.
In short they were looking to pull the same stunt your, should I say guest? or squatter? is doing to you.
I have one motto...No Pay, No Stay..
Good luck
Paul