I would have fun with them! If this happened in my hood I would attack/plunder that guy daily. If not strong enough, see if you have any friends or guildies around who can. Make sure to go after snipes/flips on their buildings too. Or you can just ignore the threat like the spam it likely is.
An important thing to keep in mind is that there is really only so much that player can do those that don't pay (assuming they even bother spending the considerable amount of time it takes to track that). I'd be willing to bet paying the FP "tax" would cost way more than any losses to plundering. Either way, make them work for it!
That said, the player is, of course, perfectly free to make demands of his neighbors. How they react is up to them.