Is there a way to restrict coupon use to 1 per email?

I'm hoping someone knows how to do this. I'm actually stunned that a customer is doing this but my clients has offer a Free download which I put into the coupon section as the price of the item on special. The problem is because I had to put in a cost amount (unless there's a way to do it otherwise) I now have someone who is using that code to apply it to numerous download products and are getting a lot for free.

Any suggestions to restrict this would be really appreciated. I can't deleted the coupon BTW because it's out there in the marketing and we have until tomorrow to fulfill. Then it expires.

Thanks!