How to apply Spam filter to signature?

This is a loophole that spammers found on my D6 forum and has cause huge problems for me:
Spammers can easily bypass spam filter (set by custom spam module) and insert their spam links into their signatures. I tried to add this preprocessor function to my template.php, in order to apply the filter to signatures. But apparently it does not work. I really appreciate your hints to fix this.

function corolla_preprocess_signature(&$variables){
  $content = $variables['signature'];
  $test = spam_content_is_spam($content, 'signature_forum');
  if (!empty($test['is_spam']) && $test['is_spam']){
   $variables['signature'] = t('This signature has been removed by our spam filter, please contact the site administrator for assistance');
  }

}