Peroxide has been used to create a cheap coral dip for quite some time. This is the first I have ever hear it used to kill Mojano, however, it could work.
I would place the rock in a bucket of saltwater with a small powerhead and heater for a few days to see if it spikes due to death inside it. Plus this will allow you to see if they are all dead. These type of pest often have a survival instinct mode where they start splitting very fast for better chances of survival.
I find it odd for a LFS to suggest that when they should have products that are made for that. They don't want to make money?
EDIT: Forgot to note that people primary use this to kill algae on their frags. It has been noted to kill ALL FORMS OF ALGAE with a 5 minute dip. I guess it could be like QT if you dip all your corals and such in this dip first, you keep certain kinds of algae out of your tank. That would mean starting with no live rock or sand though, and adding all the life yourself.
Edited by Xcali1985 - 1/29/12 at 11:49am