Maybe basing it off of the demarits of the criminal and the number of cops he's killed(By IP), and you can only get 1 criminal per IP per 3-4 days. The more people you get under those conditions, the more "points" you get. The more points you have, the more stuff you can get. Also, how about the more times arrested, the longer any future arrests on that IP(NOT THE CHARACTER) last. To prevent people from being a criminal just for the heck of it, but still get caught alot.