Just so you understand what GNU license means:

"

To protect your rights, we need to prevent others from denying you these rights or asking you to surrender the rights. Therefore, you have certain responsibilities if you distribute copies of the software, or if you modify it: responsibilities to respect the freedom of others.

For example, if you distribute copies of such a program, whether gratis or for a fee, you must pass on to the recipients the same freedoms that you received. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights."

If they copied your listings word for word, that's a no-no, but you can't stop them from re-distributing the software itself - you would be in breach of the license granted you if you tried.