denying rights?

[yawnmoth]

I was looking over the source for a random GPL licensed software
product and noticed that the authors of this product had added in a few
exceptions.  ie. after they mentioned that their software was licensed
under the GPL, they said that "person x" and "person y" couldn't use
it.  My question is...  can this actually be done?

If not, is there a link that can corroberate this?  Like maybe a
specific section of the GPL or a specific FAQ entry? (this question
didn't seem to be addressed on the GPL FAQ I looked at...)