Sometimes even the best-laid irrigation systems need a little tinkering. Maybe you have a sprinkler head that's spewing water right at a fragile plant that's in danger of collapsing every time the water gets turned on. Or maybe you have three active sprinkler heads flooding a patch of ground where really one or two sprinkler heads would suffice. So what's a poor gardener to do? That's simple: You shut it off.
Locate the sprinkler head you want to shut off. Sometimes this isn't as easy as it sounds; sprinkler heads can be hidden by vegetation or lawns, and you'll need to feel around to see where it is.
Put on your garden gloves and carefully clean all debris out of the top of the sprinkler head. Do what it takes to free up that tiny screw in the center of the head.
Turn the tiny screw in the center head counter-clockwise with a screwdriver until it feels tight, which generally will take between a quarter and a half turn.
Run your sprinkler system to make sure you have properly turned the head off.
Remove it and replace it with a screw-on cap instead of turning the sprinkler head off, if you decide you want a permanent solution. Caps are available at most gardening centers and home improvement stores.