Fight for stalemates, don't win, don't lose. Cripple their ships but don't actually kill them. Every turn the AI likes you a little bit more so eventually they'll offer you peace.
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Also, the AI doesn't like if you have ships or colonies in the systems he has claimed (it has claimed??? ).
Anyways, if you have any ships parked in his (its?) systems, move them out. However, if you think he has claimed a little bit too much of space (including some of your colonies) then it is time to kick his sorry butt.