I found hackers.

Just recently I asked my friend, if I could join his club and I requested,so about a few minutes later they accepted my request, as I went through the list of all 198 cars, I saw glowpaint cars, and some cars that aren’t even in the game, like the Nissan Leaf, and the Ford F-350 and as I am now leaving this club, due to most of the cars being hacked/nodded I have come here to tell Turn 10 about this little modding problem. In a few hours I’ll get on Forza horizon (Xbox 360) and get the clubtag. If that is Ok, or you can go see for yourself, by getting on and asking to join the club and then bust the hackers. I hope this thread helps you get rid of the hackers in Forza Horizon!

You can file a complaint against the people who have got the glowing cars, I just saw two players (one on Beaumont and one on Carson Warehouse) when I was playing Infected recently and this seems to be happening on a regular basis in random lobbies in Forza Horizon, or you can send an email to forzafb@microsoft.com with the gamertags that you are reporting and full description of what has happened, however doing it that way may take a while to recognize your message to Turn 10.