Like Ipstenu said, you may have granted people the right to act like this. I looked at your source code and you correctly claim to use the GNU General Public Licence (but fail to attach the licens file, which you are required to do - it's here: http://www.gnu.org/licenses/gpl.txt).
If you look under section 7b you find that you may "supplement the terms" with among other things:
"b) Requiring preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it"
I can't see that you did.
So it's nasty in every way, but perhaps not illegal (I'm no lawyer) to do what he/she did. I do however second that the plugin should be removed for ethical reasons alone.