Well that banned people over Bleakers for colluding as it was done in a way which breaks the spirit of Cyrodiil.I've waved my fist about this many times. Or its sinister companion - alliance switching for nefarious means - which needs to burn.
However many will come here and say that two alliances colluding is not cheating. Its playing as intended. Personally I say that's BS and this is boosting.
So report them in game if you think its boosting as that is bannable I think.