I know you've seen me talk about this before, Arran. But since you brought this up I'll share my experience of being deathmatched.
Rustlers.
Their minigun cannons are deadly to vehicles be it civilians, cops, criminals or emergency.
Some of the above mentioned teams should not be able to damage each other. For example:
Cops can not damage anyone but wanted criminals
Criminals can not damage anyone but cops.
Civilian and emergency can not damage anyone, including themselves.
But, as you saw from my complaints, whining in #CIT(which I know was annoying and I appologize for that), that the above case is not true and we experience deathmatch in such a manner.
Again, like before, I personally don't have any concrete solutions for it.
Image as seen in the complaint I made recently against the guy who did this:
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(http://img13.imageshack.us/img13/9060/rlbv.png)