Allow only 1 discount?

Does anybody know how to get product cart to only accept 1 discount? I know there's an option in the 'manage affiliates' page, but this is already set to only allow 1 discount. The problem we are having is that the affiliate discount is subtracted from the total and then a quantity discount of 10% is taken off if the customer orders more than 3 items. What would be the best way to remove the quantity discount if an affiliate discount is already applied?

There is an option on coupon codes which doesn't allow a coupon to be used with other coupon codes BUT that doesn't apply to category and qty discounts or to affiliate discounts. This would probably need to be custom programming. The potential problem is that there may be some items in the cart that don't have a category or quantity discount applied and the customer may want to apply a coupon to one of those products. Programming for that scenario, if needed, can be tough.