Manual:$wgReservedUsernames

Details
Array of usernames which may not be registered or logged in from.

Maintenance scripts can still use these.

Extensions can use the  hook to add to this array.

Prior to 1.13
The double redirect fixer has been added in 1.13.

Prior to 1.9.0
Prior to 1.9.0, the following default value was present:

Changing the default values
Overriding the default with a bundle of different user names and discarding the user names set by default is not recommended.