Again on the diplomacy screen. Click the first option, to suggest a trade/action/ agreement. click active deals at the bottom... if you have signed agreement in effect it will tell you how many years/turns are left.... If it is still in th first 20, you cannot end it without getting a bad rep.
Looks like it is intended to penalize un-honorable actions. Unfortunately, the AI seems not to be penalized, or just doesnt care.