My forum is starting to get over run by spammers who are real people registering, they bypass the recaptcha and a knowledge required security question, then post a few vague things to topics, like "that's cool" etc before eventually posting an advert with a link in.

They seem like real people doing this and I don't know how to counter it, any suggestions?

Moderating the first two posts of every new member and blocking the IP once they posted spam. They will eventually give up. 

You can use Automatic Content Moderation option in your Admin Panel for that. It will help a lot.