Currently there is a solid refund system in place through national laws that are voted on and passed by council members of each nation. However this reimbursement to bankers is only given when a player goes inactive from the game.
I would like to suggest the ability for a banker to take legal action against an active player who refuses to pay his loan. My thoughts are once a loan reaches 30 days past the last payment date made by the borrower, the banker would have the option to "Garnish" the borrowers daily earnings at a percentage, say 10% for arguments sake. At each daily update, the 10% would be automatically deducted from the players earnings (just as the ship tax currently is) and directly applied to the outstanding loan. Once the loan is paid in full, either by the garnishment payments or the player manually paying off his debt, the garnishment will be automatically removed. This would encourage players to repay their loans as well as give the bankers more security and a greater desire to give loans to those seeking them.
I understand other thoughts have been given in the past and the response was to go out and collect, however typical bankers don't just jump in a ship and take the fight to a bad debtor.