Actually... looking at 2.03 - in the admin CP - under the top options --> change options -->User and registration options --> Illegal user names :

Enter names in here that you do not want people to register. If any of the names here are included within the username, the user will told that there is an error. Separate names by spaces.

Is this not what your looking for? Don't think a hack is requird for what you want here. Hope this helps. Let me know if this is what you were looking for or I'm just confused.