Bukkit

In Bankcraft players are allowed to run into debt. The amount of owed money as well as owed experience can be configured in the "config.yml".
If a players banked money or experience falls under 0 he is considered a debitor. If he is online he is assigned immediately to the "debitor" permissions group (the name of the group can be changed in the "config.yml"). If offline, he is assigned on login.
Same procedure is applied when he pays his debts and banked money AND banked experience are 0 or higher again.