I agree that taking the keg out of the refrigerator/keezer and relieving the pressure repeated will work.
There is also something else you can do after removing the keg from the keezer. Relieve all the pressure on the keg and tap on the side of it with a rubber mallet. You don't have to hit it very hard but the vibrations from tapping on the keg will bring some of the CO2 out of solution. Think of what happens when you drop a can of beer and they try to open it without letting it sit for a while. We used to do this to the bright tanks at the brewery I worked at when we accidentally overcarbonated the beer. You may have to do this two or three times before you get the carbonation down to an acceptable level. Just make sure to purge the pressure each time to prevent any gushing when relieving the pressure. If you are concerned about foam coming out of the relief valve, attach a connector to the gas-in in side of the keg with a hose on the end of it going into a bucket. If you didn't overfill the keg, you should only get CO2 to come out and no beer. If you did overfill, just take a glass or two of beer out of the keg before tapping on it.