They should honestly lose assistance and lose at least one hand.
Let's start with a part of a finger. As incentive to work. Kind of hard to work without a hand. Still can just fine with one tip of a finger removed.
You endorse the yakuza method, I see!
Efficiency matters. Depending on the crime an entire hand makes sense. That also makes them half as useful. A bit of a finger? They can still do basically anything needed and sends a message.