You can report their conduct to the police, but the police response will depend on the laws of your state and what they are saying. If they are making threats, it becomes much more likely that the police will investigate. If it's just nastiness, but without threats, it may not even be illegal -- harassment laws vary by state.
You can also report their conduct to Facebook, to see of they'll act under their terms of service. You can also block them on Facebook.

