I actually popped up with the same question a few weeks ago
I've been playing around with removing the glitter from the blind bags since then.
I have only tested this method on these three ponies, so I don't know if this works every time but I started off putting all three in to a bath of nail polish. I put all three in to a cup of nail polish, then took them out one at a time and gave them a quick dunk in a cup of clean water and just started to rub at the glitter; the glue would ball up and could be peeled off. When the pony started to dry out or it was no longer being effective, I put it back in to the cup and took out the next one. The only problem was the next morning there was some odd discoloration to them. So I boiled them in some hot water. Getting them wet changed them back to the original color. I then, since I was doing science anyway lol, poured enough 100% pure acetone into a plastic cup to cover a pony and then swirled the cup like mad to keep the pony moving and the acetone sloshing over it. It took off a decent amount of the glitter I couldn't rub off, and did some REALLY weird stuff to the remaining glitter. It's almost like it crystalized. But by using a flat-head toothpick you can actually scrape this crystal stuff off. I found that a toothpick is soft enough that it doesn't hurt the plastic.
I'm sure you could skip steps and go straight to an acetone bath, but I figured I would layout everything that I did. Here is what they look like at current, though they aren't 100% finished yet:
Twilight worked out the best; there are fewer curves on her making it much easier to scrape off the glitter.
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login Pinkie Pie's Hair is really hard to try and get everything off of. So many curls and corners!
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login You can kind of see what I mean about "crystals" forming here; that white crust along her wings/back is what appeared and what just scrapes right off after an acetone bath. Twilight was covered in this stuff but turned out perfectly fine.
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login Sorry for being so wordy! Lol. I hope this helps you out or gives you an idea of what you can do/where to start. If you find an easier or more effective way to de-glitter them then please let me know, lol. And like I said; I have only done this method with these three ponies, so I don't know if this reaction repeats 100% of the time.