You can remove any debts for a student by removing any meals assigned if these should not have been assigned.
This can be done via Admin > Meals > Individual Student Lunches. Change the date range to that which is required, select the student and choose.
Once loaded if you scroll to the bottom you will have the option of "Bulk Actions All Date Range" if you click onto remove on this box where any meals are assigned this will remove the meals and therefore the debts created.