some modifiers to take into consideration:
GalCiv3AIDefs.xml: <FactionPowerRatioBeforeAskingForPeace>0.4</FactionPowerRatioBeforeAskingForPeace>
GalCiv3AIDefs.xml: <AIContactHumanForPeaceCooldown>12</AIContactHumanForPeaceCooldown>
GalCiv3AIDefs.xml: <AIContactHumanForPeaceRandomRange>10</AIContactHumanForPeaceRandomRange>
GalCiv3AIDefs.xml: <AcceptPeaceFromAIEvenThoughIDontWantItChance>0.30</AcceptPeaceFromAIEvenThoughIDontWantItChance>
I suppose try to further bolster the power ratio to force your hand.
I had thought that it would be more difficult with certain races, but thinking about it more, it may have more to do with how negative your relations are. Perhaps with extremely bad relations, they may be more "stubborn", affecting the power ratio required. I've been at war with races where relations were "cool". They didnt hate me, loved my ideology and the fact that I was at war with their enemies. They just weren't happy because we happened to be at war. 
I do recall those races more quickly asking for peace ALL the time, and races that truly hated my guts did take a long time before I was able to ask for peace...and more importantly, I think those arch-enemy races NEVER asked for peace themselves, no matter how hard I pummeled them....
