With the help of the Manage Disallowed Names setting, the Administrator can prevent inappropriate names, called disallowed names, from being displayed on the site and being used as usernames during user registration. Community Server provides a list of some of these disallowed names, which are displayed in the page that appears after clicking the Manage Disallowed Names setting. You can also create them by supplying the name to be disallowed in the Name field and clicking the Create button as shown below:
From the above screenshot you can see that a disallowed name moderator is created that will be added in the list of disallowed names. Hence the Community Server will prohibit any user registering as moderator and display a message: Filled in username is prohibited on this forum. Please choose other username.
Now you know how to disallow a user from registering as moderator, you can also disallow names that are similar to this username like moderator1, moderatoradmin...