I'd probably shoot them an email, and publish a blog/tweet/... along the lines of "We appreciate the contributions of <NAMES> over the last few years, unfortuna

I'd probably shoot them an email, and publish a blog/tweet/... along the lines of "We appreciate the contributions of <NAMES> over the last few years, unfortunately we can no longer accept contributions from Iranian citizens in compliance with US law." The sneakiness really is unnecessary.