I am creating some discount and coupon in the website, however i don't want the customer use different coupon all in once, for example:

they can get 5% off if they buy 4 products, but for some special customer, if they put in the code they can get a free product, instead of 5% off. but now they can get 5% off and a free product.

and if they know other code the can also add in......

how can I not letting the offer combine with any other promotion?