I suggest adding a little bank,that gives credits debts.
I picture it like this:
Maximum ammount of credits you can get is something like 5 milion.
Every day your bill gets higher by 1%,meaning each day of delay is a price to pay.
Get debuff (like one from Nightmare pilligrim), that doesn't allow you to make new debts or send money via ingame mail, untill you pay your debt.
*also that debuff is account wide,so some smart guys can't place something on GTN for 5mil. create a char.,make debt,buy it,then delete the char. lol*
As soon as you get the money you have 24 hours to spend it,otherwise all the money will go back to the bank, and you'll have % to pay.
Also,for not running away from bill, you can use escrow system,like if you have a debt, you can't have more creds higher then 350k,all the creds you get afterwards go to your bill
Excuse me for mistakes, my English is not perfect.