How to fully cancel an order if the only line on the order is cancelled

We have a situation were end users will create a single line order and book the order.  Later they receive a call from a customer to cancel the item.  They go into the order and zero the quantity on the line, however this is the only line in the order.  They should cancel the order but do not.  Is there a way to automatically cancel the order if the only line on the order is cancelled?  Or, is there a constraint that can be used to require them to cancel the order?