Removing Proprietary Bits from Illegally Closed Open Source?

hahnfeld asks: "I maintain an Open Source (GPL) project which is fairly popular among commercial companies who produce proprietary add-ons for the software. Recently I found that someone was selling code derived from my product under a proprietary license. As a settlement, we finally agreed that his software (which had come a long way from the original Open Source base) will be released under the GPL. Obviously, I have plans to distribute the newly GPL'ed code from my project's site. Now that I've made the announcement, many commercial add-on authors are saying that they believe their code may be contained in the software and it is MY responsibility to remove it or they will come after ME. I've received everything from threats to insults from the commercial add-on authors, who believe the newly GPL'ed product will cut into their business. I've already notified everyone who has a proprietary add-on that I know about, and I'm planning on cleaning out anything I find. But short of not distributing the newly GPL'ed software, is there any way for me to protect myself in the event some proprietary code gets left in the GPL code?" As open source gains popularity, this issue is bound to strike another developer. In addition to seeking legal advice, what suggestions would you give to someone unfortunate enough to be in this position?