How can I turn off water flow to toilet when the water supply valve is already turned to the off position but its not shutting off the flow?

In this case, you will have to turn OFF main water valve located in front ( or side ) of your house. Replace or repair the angle stop and turn main water back ON. Hope that helps. Let me know how you did. Milo

Great advice by Milo. You can also turn the water off at the meter or pump. Replace the angle stop,(see image) but use the old 1/2" compression nut and ferrule. This will save time and hassle . Good luck, Tom