You gave legal permission to everyone on the planet to "modify and publish copies" of NUG. This permission is binding and irrevocable. You have no legal authority to restrict anyone's access to the source code, or prevent them from making changes you don't like. In the future you may want to learn among a list of other things about open source licensing before "developing" anything.
In general, yeah. Though there have been some cases of it being revocable. The one I remember off hand involved a dvd ripper that was gpled. The code was forced to have a change of ownership by a court, and they also allowed the gpl to be revoked on it if I remember right. Was like 10 years back.
In this case though, another principal applies. This was a work for hire. Part of that should have included them giving Vlad access to the repo. The fact that this was not done and programmer X and Y are associates does indicate a dereliction here . So a hostile fork using the same repo that was created as part of this work for hire is probably actionable.