How do you make peace with an AI empire?

[Gandalf Parker]

Make sure you havent claimed their system(diplomacy menu, check the BORDERS thing), remove all ships and planets from their system, send in ships for them to kill (which usually makes them feel better if they win some fights). When they are willing to accept gifts, give thm their own system back, give them star charts, give them non-combat tech such as medical or planet boosting.

Generally at the point you descibed I just demand their surrendor but I can see some reasons you might want to leave them there.

[Arkcon]

Gandalf Parker said:
send in ships for them to kill (which usually makes them feel better if they win some fights).

Now see, that one makes them madder as well, as can be seen in the appropriate AI_Anger.txt file (if you wanna cheat and know everything about the race )

Whats great is stalemates. That does make them happy. Turtling yourself in with mines at choke points, littering their space with engine destroyed hulks (that maybe easier for bbq's Version of SE4, the engine destroyer skipped shields in older Versions of SE4). Or just send heavily shielded ships with "Don't get hurt" strategy against weak targets.

The AI will sue for peace, sometimes just long enough to pass a mass of satelites.