I'm noticing a similar problem on my MyBB forum as well. A bunch of users who post nothing but they have a spam link in their signature. I had a few users post in my Vending Machine game, but their reply wasn't relevant to the previous reply (though mentioning how vending machines help people...).
My solution for now to this problem is to make all user submitted links NoFollow.
Plugin for WWW button and website link in profile:
http://mods.mybb.com/view/nofollow-website-links
For signatures and all user submitted links, there's a plugin, but it doesn't work on user profiles so I edited the MyBB core.
Open the file
inc/class_parser.php
Search for the following (it's in the function mycode_parse_url):
PHP Code:
$link = "<a href=\"$fullurl\" target=\"_blank\">$name</a>";
Replace it with the following:
PHP Code:
$link = "<a href=\"$fullurl\" target=\"_blank\" rel=\"nofollow\">$name</a>";
That way, those signature links are useless for
SEO and Google won't penalize you.