If they are in Democracy, send the Spies in to take out key happiness infrastructure: Temples, Cathedrals, Colosseums, Marketplaces, Banks. After a few turns most AIs will switch to Fundy/Commy/Monarchy. Then buy them up.
Cease Fires are different for the AI than the Human player. The only reputation that matters is their rep to you. If you can keep them at Receptive or above with tech or gold gifts they should honor it. If you let them get Uncoop or worse they will look for a good opportunity to break it, with no repercussions to themselves. The only nice thing about the AI breaking it is that if you are in Republic or Democracy your Senate will let you make war. If they break CF multiple times you can stay in war even if they try to negotiate.
There are several actions which can goad the AI into breaking a ceasefire. During a CF you cannot ask another civ to move their units out of your city radius, so wandering around in their area is provocative. Blocking a key road/rail line with a caravan seems also to irritate them. Subverting cities or stealing techs are pretty annoying. I've heard that sending emissaries several times each turn can also add up into a provocation, although I have not tried this.