Here's something else I found interesting.
I used to think that as long as the posts with spam links were deleted in a timely manner, everything was ok. However, there are a lot of spammers that have spam links in their signature, but never post (and so the links stay there for years).
I recently ran a google search for something related to chiefdelphi. What I found were a lot of results that were spammers member pages, such as this one:
http://www.chiefdelphi.com/forums/member.php?u=34231
There were also pages that contained links to member's pages, which had spam in the signature. By posting a link to a page that has a link to what the spammer advertise also helps them, and it looks much more benign on the other page. Running a
google search shows that there may be millions of things doing that. I've cleaned up some of them, but there are still a lot left.
Just one more thing to watch out for.