The robbers need to be given consequences for robbing people, even at the slightest degree.
Make it so that when a baton hits a player with demerits too low to be imprisoned, it charges them a fine based on the amount of demerits they have. This fine will have to be wired to the bank, which you'll also have to wire to accept debt so that these fines don't become absolute just because you have no money in the bank.