[support] getting the underscore to be unusable in user names

I have run into trouble with the underscore. (concerning a mesh up between drupal and mediawiki) I would like to find a way of getting the underscore on the "banned list" for drupal registrations. If I try and register a name such as "great!!!", I get a message saying: "punctuation is not allowed except for periods, hyphens and underscores" I would like to make it impossible for underscores. Is this just a case of altering something in user.module? many thanks in advance Neil