There are no rules that say you can't and Turn 10 has not seen fit to build any working provisions into the online play to restrict the game to 'sim racing' only. They don't know it's a 'sim racer'(according to you). His kids have a lot of fun playing 'demo derby' online.
My friend bought an XBox One for his kids, Forza 5 was bundled with it. I would think people that get this game would know how to race and the unwritten rules of it. They shouldn't have to tell you or anybody else that this is a sim racer not demo derby. No most have quit playing because of trollers. If Forza is a racing game, how is it that 'crashers', 'trollers', whatever you call them have 'taken over' the online gameplay of Forza 5? Are the only fifty or so 'true racers' that have posted here on this thread, the only people who actually know how Forza 5 should be played? Why doesn't Turn 10 include an owners manual with the game to explain to everyone how they are supposed to play the game?