20 layers of foil will defeat any tags.
But as for the alarms in the games, I think you're thinking of the little white acousto magnetic strips. I've gotten mixed information about them, some of my sources have said they can be disabled with a magnet, but some of them say that they take a special piece of equipment to disable. I'm personally thinking that they probably take a magnet, just because of the simplicity, but I haven't tested it yet.
On the assumption that the magnet won't work you could also potentially put a strong magnet in the same place you're planning to conceal the merch, this would, if my understanding of these things is correct, pin the metal inside the tag in such a way that it wouldn't be able to resonate with the gates, it might unpin once you take it away from the magnet though, so keep it near the magnet when you're leaving the store.
Maybe someone can tell me first hand if either of these things will work, I'll try to experiment soon and tell you first hand though.