bad boy valve
there are a couple of possible fixes . the first might be the easiest if you have a std shut off valve. you might be able to pull the valve stem and replace it. shut the water off. using a pipe wrench to hold the valve use a regular box end wrench to losen and remove the stem nut.the stem will come out take it to your nearest home depot and they should have a replacement. or if you dont plan to use it again find a good section of pipe on the supply side of the valve (the side the water goes into the valve)cut the pipe and solder on a copper pipe cap.its easy if you have a torch. if not go to the home depot and by a kit it will cost about 25 bucks and they will even show you how to solder a joint and once you do it you will be able to do alot of your own plumbing