Hot radiators in the heat of summer

Hi all, I have a 6-unit apartment building with an approximately 7 year old forced hot water heating system.

We have a high efficiency triangle tube natural gas wall mount unit that provides both the domestic hot water for the whole building and provides the hot water for forced hot water radiators throughout the building.

I bought the building in November and it heated the whole building with great efficiency through a very cold Maine winter.

However I now have several tenants complaining that their radiators are still emitting heat in the middle of this latest heat wave.

All the radiators have two pipes, one with a shutoff going in, and on the top of the outflow pipe there is a white thermostatic valve which can be adjusted from 0 to 7.

I know that the system is equipped with an outdoor reset, but I don’t know how it works. My understanding was that it was supposed to turn down the heat when it’s above a certain temperature outside.

Even with the thermostatic valve in the 0 position, some radiators are still emitting significant heat.

Can I turn off the valve to those radiators without affecting the entire system? Is there some way I should be able to turn off the hot water to the whole system without affecting domestic hot water?

There are two main feeds coming from the furnace, each with an on/off valve and some kind of check valve/pump. I assume one is for domestic hot water, but they are unlabeled.

I have someone coming to walk through the system with my next week, but in the meantime it’s 80 degrees. What’s my best step?

Thank you!