I remember giving the Gauls 50,000 Denarii for a ceasefire when I was playing Julii. They accepted but then broke the ceasefire the next turn and attacked one of my cities. Rather stupid IMO. A ceasefire should be set in stone, not allowing it to be broken for X amount of turns once it is agreed to. It would make diplomacy a bit more meaningful.

Or in a page from the Civ book, when someone breaks an alliance, all their units get tossed out of your territory.