I prefer to package beer in kegs for several reasons. Besides being convenient, perhaps the best thing about a keg is that it’s very easy to change the carbonation level in a beer. In your case, you either added too much priming sugar or went overboard with your carbon dioxide pressure during the carbonation step. Whatever the reason, the problem can be solved simply by releasing the top pressure on the keg. This method works to drop the carbonation level, but unfortunately, it’s not a real sexy technique.
If you drop the head pressure in the keg, it will slowly return as the carbon dioxide in the beer re-equilibrates with the headspace in the keg. If you have a Cornelius keg, make sure the lid re-seals properly, or you could lose a lot more carbon dioxide than planned.
After a few hours, you can hook your gas supply back up to the keg at your normal dispense pressure and check the level of carbonation. You may need to repeat this method a time or two, depending on how badly you over-carbonated your beer! Although this is a nuisance, it is definitely a solvable problem. That’s the beauty of kegged beer. If the beer were bottled instead of kegged, then you would be out of luck.