Even if you turn the CO2 down, there may still be pressure in the headspace of the keg. Flip the regulator off on the CO2 and pull on the pressure release valve on top of the keg. A lot of hissing will ensue as the CO2 races out. You might as well let it depressurize completely as CO2 is cheap. On a side note, don't breathe directly from this stream of exiting CO2, I don't know why you would, but just don't. Anyway, after you depressurize, set your regulator to about 4psi and turn it back on.
After doing this, the beer can still be overcarbonated for a while. However, I've found that once I'm on the correct serving pressure, after a couple days the beer releases some of the extra CO2 and goes back to good carbonation levels. I have never tried forcibly getting CO2 out of suspension (i.e. shaking the keg and letting off the CO2) but I suppose that is an option if you are desperate. I would note that exiting CO2 will carry off aromatics with it, which might not be good for retaining hop flavor on an IPA.
I'd say your best bet after depressurizing is to wait a couple days and let the beer naturally settle a bit. Also a couple pouring notes, if you just got the kegs. Some people when they get too much head try to pour slowly and only push down the valve halfway or so. I find this actually dramatically increases foaming as the beer gets forced through too small of an opening and gets lots of eddy currents and disturbances. If you actually push down all the way you get less foam since the stream can flow uninterrupted. I also find that if I start pouring into a sink and then move to a glass once the stream has started I get less foam. The first initial blast is usually more foamy, but once it's started it's much less so.
Anyway good luck, and enjoy the keg. Remember what you carbonated the beer at this time and remember to use less pressure on your next batch! They can be a little finicky when you are getting used to them, but now that I've had them for a couple years I would never dream of going back.