I have an idea, instead of points for killing zombies like Warband suggested, how about you have a karma system to stop people team killing and such.
EG
1000 - Karma = Normal
500 - Karma = -%20 damage or health
you get less karma for team killing, karma can increase over time or by finishing objectives or killing zombies ext..
However this is just a system to stop teamkilling, you could make it if someone had a certian karma value they would be kicked or banned.
Kaies
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