I've seen about four methods of getting that out from other threads on this:
1. Sometimes you can tap the plug head face down on a solid floor, and it may be loose enough to drop out.
2. The Sharpie method mentioned above to make it loose enough to fall out
3. Someone mentioned holding a needle with a pair of pliers and heating it up with a lighter flame, and then sticking that into the little ring of plastic so the plastic melts around it, and the needle sticks in there. Then you may be able to pull it out.
4. Similar--drill a small hole in it. Then get a toothpick and put a dab of superglue on the tip and stick that in the hole and let it dry. And then try to pull it out with that.