The AI problem is hardcoded in RTW. I have yet to see any mods address it and in fact pretty much all of them have stated in the forums how it's WAD from CA.

Limiting your borders is probably the best way to handle it since if you don't share a border - they'll nearly always make peace.

On a side note - for the first time ever playing any version or mod of RTW as Romanii I've shared a border with a now very very large Sweboz to my north. I merely have trade rights with them, have never allied with them and have always maintained large garrisons along the border (expecting the backstab). For probably 150 years we've been at peace and they even coughed up a gift at one point.

Probably thousands of games of backstabs-R-us and now this - very, very strange.