Embargoing certain countries, but still being open source

[Anton Lauridsen]

Hi

I have a couple of Open Source projects, licensed under BSD, but for
which I have the complete copyright. I am contemplating to change the
license into something, which prevents US citizens and/or interests,
directly or indirectly from using my software.

Does such a license already exist? if not, would it still be considered
open source?

If such a license does not exist, how do I go about creating one?

/anton
ps. this is *not* a troll, and please do not try to argue, that I
shouldn't do this, the decision has been made, it is merely a matter of
"how to do this?", not "should I do this?" or "is this right or wrong?"