Can a mechanical temperature controller serve as a stopcock?

As a rule, two shut-off valves and one adjusting valve are installed, as in the diagram. It matters which faucet was used when installing the radiator. If the control valve is not designed to shut off water (according to the instructions), then in this case, check valves must be installed in front of such a valve. If the control valve means the corresponding valve, then there are no reservations - it can be shut-off and an additional valve is not necessary.

There is one more point. In rooms with central heating, it makes sense to install shut-off valves on radiators only made of steel or brass. Hot water flowing through pipes, as a rule, has a very low quality, so the control valves quickly become clogged and cannot fulfill their function. And in a private house, it makes sense to set thermostatic taps. Such a faucet mounted on a heating radiator will significantly save energy.