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.

Current version
as of wmf1.20, September 2012

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: