Try to add custom question to your reg form, that will usually stop most of the automatic registrations. For the manual ones, try to use something like profile moderation (don't allow them to modify their profile only after 5 posts or don't let them post links unless they do an other action first).
2 2012-09-24 14:14
Re: Is this ok, optimising load speed with cache (7 replies, posted in Discussions)
I wondered is there any to optimize the cache for punbb? For example to use html minify, maybe combine the images into sprites etc.