Thanks for the info. I had considered the possibility of other Roman factions attacking me. But in this case, it wouldn't happen, as Julii can't get out of the Italy because of the angry britons, Scipii are no more, as they turned into rebels and the senate never leves the area near Rome. I could just destroy them all, but that is not my intention, just wan't to decrease their territory's to minimum and make them live near barbarians.

I will try the method of besieging all cities and ports on some faction later, but that means that it will be hard to contact and make the offer to them, the only way is to find their diplomat.