slow down your expansion

... make sure your Piety is high, as that helps reduce BadBoy points over time.
It is a game related mechanic, to stop the player from taking over Europe before 1200. In CK vanilla, there were no BB for taking pagan/muslim lands, so you could basically conquer all of Africa and much of the Middle East withing 60 years.
You can force vassalize which is reputation neutral. And if you let them keep their titles (you will get a claim if they rebel later), you can even get a reduction of BadBoy points. You get +.5 for declaring war I think. Then you beat up some poor count with two counties. You force-vassalize him (+0 BB) and give up the two titles (-1BB each). You have now reduced your BB by 1,5 points and gained a vassal with two provinces. If you want you can even claim one of the provinces, and you would then gain one province, one vassal with one province, and still reduce reputation.
Don't be such a warmonger ... play nice with the other children

... and rebelling vassals are a god-send for your reputation, as you can do as above and quickly get your BB under control again. Just keep reputation at slightly tarnished, and you should have little vassal problems.