You can changed the frequency of colonization events, but you'd have to do it as a mod.
Specifically in the GalCiv3GlobalDefs.xml there is the following setting:
According to the schema that governs that, it looks like that is a decimal value that is set between 0 and 1. So if you'd want it to be 50%, set it to .5. 25% set it to .25. And so on.
Now because GalCiv3GlobalDefs.xml is NOT an appendedable file, you'd have to copy over the entire GalCiv3GlobalDefs.xml file plop it in your mod folder, change that one setting and then save the file in the mod folder.
Doing a quick test, I set ChanceColonizationEvent to .5, and sure enough I didn't not get a colonization event every time.
So there you are. Simply copy the file GalCiv3GlobalDefs.xml into your mod folder, change that setting to something you like, and away you go.
Though, personally, I agree with Larsenex. Ideology is already way too hard to come by. Cutting into one of the main sources may not be the best of ideas. But, hey, that's the game is so moddable. Play the way you want, and all that.