I feel the better solution is INSTEAD of telling people what NOT to do, take a petition and approach people who have bought the DVD and ask them to sign a petition asking for the release of the original editions of the Star Wars trilogy. You can educate them about the fact that Lucas is only putting the Special Editions out on the box set. A fact that I'm sure some customers will be very surprised about and will probably get you a good number of signatures on that issue alone.
Yes, I know there is an online petition, but I work in the entertainment field and online petitions mean very little to companies. It's hard pieces of paper that REALLY stand out. I realize that the efforts of OT.COM and others on behalf of releasing the OT on DVD are noble, sincere and valiant. But having a big stack of papers plopped on your desk is much more dramatic than someone saying "hey, there are 50,000 signatures on the online petition"
In this day and age, people take the easiest and convenient way out, and that is by doing it online.
Just speaking from my experience in the entertainment industry, hard copy letters carry much more weight than emails.
If you want to bring petitions with you to for people to sign, I am all for it and I may even want to participate. You will probably be well received by customers and you may even get support of the management, because when you convince them that they will sell even more Star Wars if the original editions are released, they will be more than happy to support you; or at least not bother you as you collect signatures. If you want to accost people (yes accost) and tell them not to buy merchandise, I strongly advise against that. It would only get you into trouble and make us OT fans look bad.
Please consider the petition drive idea rather than a picket line.