Has anyone any suggestion on how to stop the spammers? I thought the captchas would be sufficient to prevent spam bots, but there seem to be ways to circumvent those.
Is there a specific mod to add more complicated captchas or something like that?

Have you come across this yet? These dudes have posted a suggesting methods to deal with the recent rash of spambot exploits.

Across many of the forums I frequent these intrusions have become pretty common recently, most seem to have addressed the issue. I don't think it's as hard to beat as it would immediately seem.

Can't you simply delete the topic they make? Of course it wouldn't stop them, but at least there wouldn't be large amounts of the spam topics in the forum.

A possibility is to allow the users yourself, manually. The problem is that some may register to post immediatly, and if they can't, they may never come back...
Also what happened to me on a messagboard is that the admin obviously forgot to activate my account...

You may give some of us the rank of "spam moderator" if you will. There's some spam, but not like 100 a day... So if several are made moderator, it'll be clean regularly. When I say moderator, I mean only to clean the spam, not to start appreciating posts, closing and merging threads and things like that...
Some of us have been around here for a while, and I don't remember any flamewar or whatever that could result in moderator power abuse...
But maybe you prefere to ban the IP adress for each spammer that registers and as such prefere to do it yourself.

I've also seen some word censors, that rather than transform a word in another one ask you to change it before posting, so anythign sex or drugs related -well, at least drugs related- can't be posted

I also thought making it mandatory to have an avatar could help, seeing how spam bots never have one, but some people don't know how to upload an avatar, sooo...

Another board, one that I mod at, has had some spam problems. not quite this much though. We usually just have mods and admins delete them on site and then ban the spambot jsut in case.

Also, maybe new threads could be started with a code or symbol. LIke "_|_ New Thread title". Then again if someone forgets it might be bypassed as spam. Just an idea.

but it's a good idea.

Better captchas for posting and registration would help, also delete any user with less than 3 posts.

Alright, now you have to enter blame related questions to join! This should keep them off once and for all.