I think long teleports are what people are getting banned for, as the server needs this information. With one of the patches, they must've implemented a server-side flag that records unusual xyz coordinate activity.
Big Head probably isn't the problem because they would need the client to report aiming data to the server, which if it did, big head wouldn't work.
Infinite Ammo, at least for Dexter's, doesn't modify clip size (EX. 9999), and if they fixed OiN, then it wouldn't work beyond the normal clip size.
Most likely, it is Long Teleports that did everyone in, or blatant use in PvE / DZ.