It is possible to achieve peace, but you have to get them to move to a friendly demeanor first, and there is a minimum time IIRC of a year or five.
What you need to do:
- Not fight battles. Fighting lowers their friendliness. Try buffer zones, mines and black holes instead
- Give 'em trinkets. If they're accepting anything at all, you could give them a few resources to help them see you're not all evil.
- Out of sight, out of mind. Don't let them see your ships over their colonies, and try to avoid contested systems. Buffer zones help in this regard.