Currently set to Civ 5 civs only. If you can, you must type in the name as it appears on http://www.carlsguides.com/strategy/civilization5/civs-leaders/.
For example, to ban the Greek civ, it would be "Greece", not "Alexander" or "Greek". After a civ has been selected, it will not be reassigned. Some civs come pre-banned (the ones we don't use in multiplayer games).