We too had a "Silicone-Happy" PO...we found that if you spray Permatex Gasket Remover on it, it will soften it. It's a lot like the Aircraft Stripper we used to remove the clearcoat but works better on silicone. Use a sharpened popsicle stick to scrape it all off, you'll probably need several passes to get it all off. Then wipe down the area with mineral spirits to remove the remaining residue. All traces of the silicone must be removed or else the new Vulkem/Parabond will not adhere It's a very laborious and thankless task...I'm sure it didn't take as long to put all the silicone on!
Permatex Gasket Remover can be found at most auto supply stores like Checker, Pep Boys, Kragen, etc.