How about we advise the user that he cannot post links for 8 posts (displayed when they first create their account).
If the user tries to post a link, they will recieve a (very clear and flashy) message telling them that they cannot do that until they have completled 8 posts (and the post will not be put up, it will wait till they remove the link and will not count as a post).
If the user attepmts to post more than (say) 3 in a row with links in them, the account is banned.
If the user misses the "REFRAIN FROM POSTING LINKS!!!" message more than 3 times, chances are almost certain its a bot (or a human with the intelligence level of a spammer).
Signatures though I've never seen as a real issue, but I'm sure we could rig up a system for that too.