Offer *limited* immunity for turning self in for SSI or disability overpayment fraud. If we never find out, they will just be using that money and if caught, they rarely pay it (or all of it) back. Plus in many circumstances, it was partly the agency staff's fault - an overlook or something.
This way, under certain circumstances (length of time, amount involved, number of people involved, etc.), some may turn themselves in.
(Remember the stealing cable tv is a crime promo?)