“pay them $1500 to get out”
I have did this a long time ago with a tenant. It is certainly not gratifying, but often tactics like this are the easiest and cheapest solution. If you want gratification, cut them (and the IRS) a 1099 for that $1500 and the back rent and let the revenuers have at them. Also, now you can deduct it as a business expense (but please consult with your accountant first!).
Great point on the 1099. Just do it after they leave and before year end.