Turn off the water supply feeding the irrigation system. Disconnect the power going to the system.
Dig around the irrigation head you need to cap until you locate the pipe connected to it. This should be within 12 inches beneath the head.
Unscrew the irrigation head from the pipe. Replace it with the irrigation cap.
Turn the water supply and power back on. Run the irrigation system to look for leaks around the cap.
Fill the hole back in with the removed dirt, and then mark the location if you plan on using it again.