I've lied before and said something along the lines of, "I'm sorry, we can't take these. They're fake coupons that have been circulating online like on Facebook. I know it's confusing because the people posting them don't know they're fake."
Basically it's like calling out the guest by saying you know they're fake, but making it seem like they made a mistake, and thus not blaming them. Super risky, I don't recommend it, but in certain contexts, it's been useful. Just, pretending you've seen them before, and being apologetic because you think the guest was sent bogus coupons....even though you know they either made or bought them. Lol