[Q]

Just for curiosity dumbluck: which races got angry after you had a treaty with them?
In my experience this varies very much from race to race. Some races behave like you described, other maintain the treaties and their fiendly mood. And I think you can change that by modifying their anger text files.
One problem that I personally find much more frustrating is the faked "declare war" message.
There is absolutely nothing you can do against it. The AI will reject every new treaty proposal for over a year and propably until then there have been so many battles between you and the AI that his mood is murderous.
Among human players this would be settled quickly ("Hey the Last message was faked, let's stay friends). In my opinion the effect of the faked message should not be a full state of war, but just an increase of the anger level.