Why are some players "banned"

I have come across several players who have been "banned". These guys would make nice farms, but you can't raid their villages. So the question is, what causes players to be banned in the first place, are these bans temporary, and if so, how long is the ban period?

Multi accounts, more than likely. If this is the case, they won't ever be unbanned or attackable.

Other infractions can lead to them being unbanned at a later time.