Discount System - Have the ability to make discounts be Exclusive

There should be an option to make it to where the discount system does not stack if we do not wish it, i.e when a customer uses a coupon code discount of 3% and it stacks that discount on a 2% automatic non-coupon code discount giving a total discount of 5%. Something like a checkbox that makes this impossible for the end customer to coupon code discounts, where a coupon code over-rides an automatic one.