[Bluemax712]

Have you tried this?
ACP -> Settings -> User Registration Options -> Illegal Usernames

You can add XYZZY to the list and then any new accts using it won't be allowed
like: 123XYZZY or 123XYZZY456, etc.

the setting won't affect any currrent accts. using it.

Edit : sorry above settings are a VB4 option - not sure if same in VB3
Edit2: looks like vB3 also has Illegal Usernames too

I found a whole bunch of fake accts using names like forumadmin, forummoderator, root etc. in a server before adding to them to the list