Supply stop valve isn't, well, stopping the supply

[Zakaroo13]

Was attempting to set up my bidet at my new apartment in a very old building. Cranked the stop for the toilet until it should be good and closed; she's still flushing. Tightened the central screw-still flushing. The supply is an exposed pipe running along the baseboard, and it gets tapped for the bathroom faucet before reaching the toilet, so I thought...nope, the toilet still flushes and the cold water still runs. And I am entirely unsure where the shut off for just my unit would be. Any advice would be greatly appreciated by not just myself, or my un-bideted bottom, but also my new neighborhoods who I'm sure have already grown tired of me loudly chastising the toilet. Included pics that I hope illustrate the setup.

 

[arctic bill]

go look around the hot water tank and see what you find

 

[Twowaxhack]

I’d call the building maintenance and have them repair my toilet valve and maybe show me where my main water cut off, if I have one.