I know this may sound dumb but, if you turned off the main water supply to the sprinklers and water keeps flowing out of this one head, is there a chance that the sprinkler head may belong to the neighbor?
I would turn off the main valve that comes into the house on the inside and then go check. Now this would be the very first valve that would be just before the water meter, (if you have a meter). Turn your sinks on after and see if any water comes out with force. If it does your shut of valve may not be closing all the way.
The head may be bad and stuck open but, is this head at the low end of the system and is just draining the pipes after the water was shut off?
Worst case, I would turn off the water tonight, turn it back on just for showers and coffee tomorrow then back off again, and call the sprinkler company on Monday.