[Suicide Junkie]

AIs won't accept any treaty for some time after a declaration of war. I'm not sure about the timeframe (couple years?), but battles do not extend the time. DO beware of intel ops, since someone else could be sending fake DoW from you, and thus extending the no-treaty time.

If you really want peace, fight stalemates. AIs seem to like stalemates, and the original ones at least will become friendlier.
Sending an escort against an enemy colony will do the trick.

Once they are friendly, all you can do is keep asking for treaties. Offering treaty trades is probably not worth it, just go for a straight treaty offer, and let the AI know there are no strings attached.

If all else fails, use WP manip to close all warppoints between your empires, cutting contact, and then reopen them.
You can start from a clean slate.