Marc,

I sympathise with you.

I've had a little play with PHP before and might possibly be able to help. What database is behind the scenes. I'm guessing it's MySQL. If so then

No doubt you have some admin functions screens available only to you as administrator. Maybe it's possible to add a couple fo admin functions (buttons?) to one of those screens and fire off som code along the lines of

1) Remove the URL from any users with less than 'n' posts; or
2) Delete any users singing up more than 'n' days ago with no posts.

Maybe this would help clear out the trash.

P.S. Think i hit the wrong button somewhere as this message looked like it posted itself while I was halfway through writing it.