Be carful when deleting users from a DotNetNuke site. You should delete all the user's "owned" content (such as the users Blog) before deleting the user, or you might end up with "orphaned" records in the database. You may then get the infimous "Object Reference not set to an Instance of the Object" errors :(

 A safer approach could be to "Unauthorize" the user instead of deleting it. This will also prevent the user from re-regestering again with the same credentials.

Note: To be fair to the DotNetNuke developers, I'm not claiming that this "orphaned records" problem is very common, but I have had it happen to me (when deleteing a user that owned a blog). A good recomendation, as always, is to make backups before doing anything that could cause problems.