This is quite easily done with a filter plugin. Here's an example similar to what I use on my site:
function filter_handle( &$handle, $olduser )
// allow admins to have whatever name they like
if ( qa_get_logged_in_level() > QA_USER_LEVEL_ADMIN )
if ( preg_match('/admin|sitename/i', $handle) )
return 'Sorry, that username is not allowed.';
That matches anything with "admin" or "sitename" anywhere in the handle.
Hope that helps!
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