To prevent Clients from going into a negative balance, navigate to System > Settings > Accounting Settings > Client Balances, and enable the Prevent negative balances setting. This will stop any Lessons from being marked as ‘completed’ if doing so would cause the Client’s balance to fall below zero.
Prevent negative balances setting, highlighted.
If you prefer not to restrict Lesson completion, you can instead enable Low Balance Notifications in the same menu. Set a Low Balance Threshold and TutorCruncher will automatically notify the Client by email if their balance drops below this amount.