So another feature request if you excuse the spamming but after my first weekend really making an effort to play online, wow.
Summary: Collision toggle per game in options and per player on user menu. Basically, collisions 'by consent'.
I guess on other games it's less noticeable or more ignorable but it's frustrating to set up a hardcore mode one star game online and playing for hours with someone only to have some idiot jump in for five minutes with no plan other than to find the biggest unlocked truck and crash into, flip or disable everyone and leave.
If there was an option to select 'collisions on/off' on the overall in-game menu, and in-game, per player, then you could easily and immediately take the troll's reason for joining away from him. If he wants to keep playing, or just mess around with other people wanting to do same then fine, but if you actually want to 'run a map' with others that do, especially in a challenging way, and still have the realism and value of collisions from people who aren't looking to spoil the game (eg by nudging you out of potholes, helping you from slipping away in the river etc). Then you still can.
My ideal implementation would be in the game options have a tick box for "Multiplayer collisions on/off" which would define the default behaviour for you on game start. You could also have the choice in the game options. Then you must still have a choice in game to override per player from their player icon (or to request collisions, as long as the menu pop up didn't stop you from doing other things). Or also collisions off/on/on for friends & party
What's important is that you could play with collisions on with serious players, but basically cancel out the idiots.
You could even go one step further and 'troll mute' per player, which would basically mute their sound, headset feed and makes them invisible to you. ie one step further than the ghost overlay.
This doesn't stop anyone from playing but does stop people coming in with the express intention of messing up somebody else's game. It also means if someone just wants to goof around then fine, they still can with other people who also want to, without getting in the way of people who actually want to play seriously. Basically take a trolls toys away from them and they will leave.
A few people have mentioned kick controls and 'vote to kick' which is one way to look at it but this doesn't always work because not everyone votes and you can't vote off a hosting troll, or a bunch can vote against a fair player. I'm all for 'kick player' if you started the server, but would also suggest adding a feature that on kicking, removes all the points (ie garage/logging etc) earned by that player in the game. This would avoid someone putting time and effort into unlocking garages and logging etc only to be kicked because the game host wants to 'finish' the level once the hard stuff has been done for them.