That's a term borrowed from pop culture cliché, most prominently 소년만화 (boy's comics, or shonen manga as they call it in Japanese) - if your Korean is good you may find more information on this article on 봉인 (check "대중매체" section).
Usually the archetypical story goes like this: thousands of years ago there was a great evil spirit that terrorized the world, which was finally defeated by powerful magicians, but the evil spirit was unkillable, so all they could do was bind the spirit with very powerful magic and keep it dormant in the deepest place of the underground dungeon, and now our hero/villain/whatever somehow stumbles upon/barges into/murders his/her way through the palace, get their hand on the magical seal (봉인), and releases it...
Similar themes also apply to magical superweapon, or your super-instant-kill skill that was inside your soul all along the way (you just had to look for it!).
In short, 봉인해제 would mean something like "Unleash the power!" In the particular context you mentioned, I fear they mean the power to eat as much as they wanted. Oh well.