If it's just a plug and you are going to replace it with another you can try to either hammer in a larger sized socket, that might give you enough grip to get it turning to take it out.
I think this is what I will try to do. I've got an impact hammer, but no allen head sockets to go on it. I'll have to buy some and try this out and see how it goes. Wish me luck!
2005 GTP - AL 605's gapped at .055", SD headers, WAI with 9" cone air filter, 180 degree t-stat, green coolant, otherwise stock.