Have you ever received a client from a coupon site? If there's one you don't like, you can always petition them to remove your listing.
Here's my thing about the coupon places and especially those offering the 50% and 75% off. Do you really want a client that purposely set out to look for "coupon" so they can pay the minimal amount on things? Often, those are the clients that are going to cause the most amount of problems.
I would make sure that the word coupon is nowhere on your site, and improve your own ranking on various other sites to push down the listing for the coupon place with regards to your name.
But again, if you're not getting clients from them, there's limited things to worry about anyway.