revoke GPL?

Several of my clients/employers received spoofed emails this weekend announcing that "The Zen Cart Company" has revoked their license to use the Zen Cart software and demanded immediate closure of their sites. Although it was easy to demonstrate that this was just a prank, I've been asked to find out with certainty if the real Zen Cart folks could selectively revoke permission to use the software by any one person or organization and, if so, precisely what actions would trigger revocation of license. The concerned parties have been provided with a copy of the GPL for their own scrutiny, but remain concerned.

Thanks

Oh brother ... Once code has been released under the GPL it can NOT be revoked.

Please PM the details of who and where those emails came from, thanks.