I think the system that was proposed is a bit too broad, and since this happens rarely, I feel it's more likely to punish people with good intentions than those with bad intentions. This has gotten me thinking about a possible other system though: if you're a civilian, and you're a passenger with a member of either team, and you shoot someone of said team, you will be auto-kicked out of the vehicle. So, for example, if you're a civilian riding with a security, and you're shooting another security or police, you will be kicked from the vehicle. This won't prevent all of the abuse you described, and will still allow people getting away, but I feel it will be more efficient in punishing those with bad intentions.