This is a suggestion on how to combat spammers.

The traditional methods are stuff like capcha or odd limitations that you have to post a certain amount of posts before you get to post a link or picture and so on. Problem with those solutions is, that they annoy all the real, genuine posters while the spammers usually find a way to bypass it anyway.

So how about looking at behavior instead?

For example, spammers often post 15-20 posts in less than half an hour. Real people rarely do that, unless they use the forum for some sort of chat room for one-liners or something.

Spammers typically post the same link in multiple posts. Real people rarely do that if ever. A regular guy might post a question about buying some brakes and a link to the brakes in 2-3 subforums, but not in 10 posts or more.

How about implementing a tool that catches all posts from a user and/or ip number if the user posts 10 or more posts withing 30 minutes or if a user posts 5 or more posts that are either identical or contains identical links. Posts like that are moved to a special quarantine area for an admin to evaluate. At the same time all following posts from that user/ip within the next 24 hours goes to the same quarantine area unless an admin white flags it.

However don't make the exact rules for the filter public, if a spammer knows that 10+ posts in 28 minutes is a no-go, he'll adjust his software to add bigger delays.

Now once in a while a regular, real, honest poster might get caught up in the filter, for example someone being real ecstatic about his new bike and "spamming" a number of subforums with the great news or someone will do a bunch of posts in a short timespan in the test forum to get the postcount high enough to attach a picture (perfect example of an ineffective rule that annoys real people and don't stop spammers anyway). So you happen to be a happy camper and accidentally end up in the quarantine area by mistake, but the admin looking at the quarantine area will spot the problem and white flag you again, putting your posts back where they belong. If you happen to be a speedy poster that frequently end up breaking the "posts per hour speed limit" the admin can give you a special flag that tells the quarantine function to ignore you despite posting at a spammer-like pace.

In short, a filter based on behavior, lots of posts in a short time, lots of posts with the same link, stuff like that. If the filter judges a user to behave in a spammer-like way, it quarantines the user and the posts and lets an admin give the thumbs up or thumbs down before the posts are made public. Anyone else posting at a slower pace, making posts that aren't all identical and include identical links will never know the filter is even there.