I used to get really pissed off while driving. It wasn't even just when someone was a dick like what you described, but going to slow, not stopping at a stop sign, camping in the fast lane, you name it. While I'm not perfect, I decided a while ago to let that all go, drive defensively, and just try and relax as much as possible when I'm driving. Getting angry at some complete stranger for driving like a moron isn't doing anything to them, just me. It was putting me in a terrible mood when I'd finally arrive at my destination some times, no to mention being angry like that wasn't good for me physically. The biggest FU you can give to a bad driver is to simply avoid or deal with them while they're near you and know that the consequences of them driving like that will likely catch up with them. Whether that's tickets, accidents, road rage incident, or worse, they're inviting misfortunate upon themselves if they continue to do that.
