A jail function is nice. If someone steals there is a xx % change to get caught and go to jail. Now that is one thing. It would be hilarious cool to see which users are in prison, so to do that it must be visible. A police cel with in it or behind it the avatars reduced to approximately 50 % or so behind bars. This requires some specific grafics but it's worth it i think.
When in jail thou can not post nor reply for a specific period of time, altered in the admin cp eshop settings but alas...you could buy an escape. That escape could give a random result of success and costs $. Or...others can buy you free. When in jail the user(s) could have an avatar forced in their postbit..a jailbird with striped outfit and a jailnumber on it.
To make thievery realy attractive their must be something to winn for the thief also. By a certain successrate the thief could be rewarded with a special gift, a crime statue or something to show that crimes pay if you are lucky enough to live the day.