The FireBug forums were rather heavily spam infested recently. Most of it was fortunately in the rather boring General section, but it had spilled into a few of the other categories too, so I asked Joe Hewitt if I could help out blasting it to bits (for the benefit of the community -- it's a small service to offer for a project you really care about), on those occasions I peer through the forums anyway. I could.
Fighting spam on a phpBB board is painful, as it's probably about as much work getting rid of it as getting it there in the first place (for the spammers); lots of clicking and waiting for page loads and the like. I had anticipated this lack of convenience, though, and was prepared to rewrite the phpBB admin interface to make it more workable.
Thus the phpBB quick purging (direct install link) user script was born. If you have logged in with admin rights (the script knows, by the presence of the link "you can moderate this forum"), it adds delete links to all posts or threads in view, and a "delete all" link at the bottom. Enjoy!