what if i am a follower of the seven and I don't like the guy burning people; I can't revoke his title even though he's lighting peasants on fire
If your r'hllorite vassal happens to be zealous, it will be a short wait before they try some plot and you can emprison them. If you fail to do so, it turns into war and if you win and the realm is stable enough, you can revoke their lands.
If your r'hllorite vassal is not a zealoth, though, you can have them at your council in a mission that will keep them at your court and have your septon doing the rounds in trying to convert infidels to the Faith in your court. It worked smoothly as silk for me in turning half of the North to the Seven in 50 years in a Manderly playthrough in which the Starks took the Iron Throne and confided the North to my dynasty.
But if you reaaaally hate R'hllor, is not above intrigue and can't wait, you can try to start a plot to forge acusations against the troubling vassal, giving you an excuse to imprison them.