How to disallow certain strings in signatures? As I mentioned here, spam filters do not apply to forum signatures. This opens the door to human spammers to put their spam content in their signature. I am wondering what is the easiest way to tackle this problem? One obvious solution is to extend the signature or spam modules to fix this loophole. But that is beyond my coding capacities as I know very little of drupal API. 

So I am wondering whether this can be done using some sort of preprocessing or defining some rules to check the signature for certain regexps and disallow registration/saving profile when the string is flagged as potential spam? I really appreciate your hints as I'm badly suffering from this problem.
In drupal, whichever field you are taking input as, you need to make sure that only plain text is inserted by the user. This way, drupal will run check_plain function to the text added, and no links, xss attacks can be made.