So far, the only way i found to at least slow things down a bit in the spam department is to make sure search engines don't index your site and clearly state so in every page.

Spammers want their spam to show up in search engines. So suddenly your forums seems much less interesting. Of course there are a lot of downsides and it wont work with automated bots.

I'm currently also experimenting on ways to detect and eliminate spam bots. But i probably have it way easier, since the site i'm doing this on is neither a) of any signifcant relevance (so it doesn't matter if it's down for a few hours while i play with it not b) do i use a widely spread forum/blog/whatever software (wrote my own webserver). And since the site also isn't commercially relevant in any way (just my personal blog, where i want to add a discussion system), i have no bad conscience when "playing with my food", either.

If you trying to stop spam on an existing, well known forum with lots of users, it's much harder. You don't want to experiment too much (downtime=bad), it uses a well documented software that is easy to automate from the client side and it's probably a valuable target for spammers (well known, google-indexed, high reputation because many site link to it). For very big forums, even hiring people to manually type in Spam can be profitable, so it get's harder and harder to detected spammers...