If you have no other leverage...
...then small claims court. Have only had to resort to this twice, but have always won.
I think it's pretty ballsy that someone that refuses to pay still expects you to show up. But they probably only do so because they believe that you're soft enough to actually consider it. I've had this happen a few times. It's really a sign of disrespect, and if there's one thing that I've learned about successful consulting, it's that it only really works well when there is mutual respect.
Of course, when you go there it may seem as though you are burning bridges. But when a client refuses to pay, the reality is that they're the ones who've set the fire. You're just acknowledging it.
