All you can do is to shut off the gas, then "burp" the keg by popping its pressure relief valve. Do this frequently for a day, then pour a sample and see how its doing.
Note that you may not even have to hook up and turn on the gas to draw a few ounces to taste - if you don't pop the PRV before you draw a sample there may be enough internal pressure in the sealed keg to do that much on its own.
But if you do have to hook up the gas, make sure to set it low, and shut it back off as soon as you draw a sample, lest you prolong the time it's going to take to tame that brew.
Eventually you'll have degassed the beer down to a suitable level, and you'll be good to go. Just keep an eye on your regulator as it may have been creeping on you...