If you have a one-pipe steam system (the most common in smaller residential buildings in the city), there is another way to effectively stop a radiator from heating that doesn't involve opening and closing the valves. You just have to block the air from escaping out of the air vent with duct tape. This can often be just as effective as closing the steam supply valve, but you don't run the risk of creating leaks around the valve stem by using it repeatedly (the valves ARE meant to be used, but only for maintenance purposes, not opened and closed over and over again).

Beware that doing this may cause reduced heat output and / or increased hissing from other nearby radiators because the air in the steam lines has one fewer place it can escape. As long as every other radiator has a working air vent this shouldn't significantly harm the overall effectiveness of the system, though.

If you don't know what part of the radiator the air vent is, this article has some nice illustrations: https://www.sparksuper.com/5-steps-to-healthy-steam-radiators/

Also consider just telling your super / landlord the heat is on too high. Talk to your neighbors and see if they're hot as well. Sometimes they just set the central system really high assuming somebody is always going to be cold and everybody else can just open a window. If everybody is hot, though, that excess heating is just wasting their money.