The only thing that I can see that might work for the valve is the obvious WD40. But in case that doesn't work, which it probably won't, is to shut off the MAIN water valve, then flush the toilet until ALL the water is gone from the tank, as well as the bowl. Then you will have to replace the toilet shut off valve (don't skimp on the type of valve either). I hope you DO know to becareful of the sewer gases that can come up from the pipe. Good luck.