Managing Customer Churn using Commissions

I want to motivate sales reps to minimise their customer churn.

We use 'open' sales orders to bill subscribing customers.  Sales orders have a blank end date for as long as the customer subscribes.  Sales reps are commissioned when new sales orders are approved (effective date for commissions = sales order date).  We use Alt. Sales. as the basis for commissions.

When a customer cancels, we close the sales order and enter a date in the 'end date' field.

I want to be able to use the loss of this customer to count against the sales rep in the month of cancellation (to motivate the sales rep to try to retain the customer).