Of course you can fix it! What's the temperature of the keg now? If it's room temperature, the co2 will come out pretty easily. Start by taking it off of the gas, of course. Then pull the pressure relief valve often, releasing all of the gas in the headspace. It should get to a "normal" carb in a couple of days.
While OFF THE GAS, purging will eventually get you back down to where you want to be. Yooper is spot on as usual in that it will off gas faster if you let it warm up.
Bright side, that beer has some SERIOUS head to it!
When I did that I just shook the hell out of it, then pulled the safety valve with the keg in the wash basin. Then afterwards sprayed the keg off with hot water and checked carbonation again. It was back to normal after doing that once.
I heard from my LHBS that if you drop the keg on the floor from about six inches height that the carbonation will come out of suspension quite readily, I haven't tried this yet though.