Suggestion 1:
If a player takes part in too much crime in a city, ( Even if he is killed every time, it builds up ) He will be temporarily locked out of the city for oh lets say.... 30 minutes? ( Player "shithead" is continuously throwing plasma nades at people and after the 5th time he is kicked out of the city for 30 minutes ). Btw this does not affect other cities, it is entirely seperate.
Suggestion 2:
If a player takes part in too much crime in a city, he will be ungaurded, as he will be considered an outlaw and must either wait outside of the city to stay alive till the 30 minutes are up or he can hope nobody recognizes him as a tagged criminal. After the 30 minutes are up the amount of crimes till tagged will be completely refreshed. ( This is continuous until the 30 minutes are up, they cannot just be killed and come back ). He basically has to wait till his heat cools down.
Suggestion 3:
If a player takes part in too much crime in a city, .... someone else fill this suggestion up I'm out of ideas.

I suppose with this you could add an outlaw waiver if suggestion 2 would be implemented, Listing all the people who are tagged. This would, ofc, be a constantly changin piece of paper :/.