Is it illegal for the property to turn down the heating flow?

It's not illegal, but there are penalties.

According to the regulations, each property service enterprise with the task of transferring heating should check the equipment in time to ensure the timely opening of the gate for heating. During the whole heating period, the valve shall not be closed or turned down at will to reduce the pipeline pressure and heating quality. If the realty service enterprise maliciously reduces the pressure of valves or pipelines during the heating period, the district realty office shall promptly investigate and deal with it, and give a warning, make rectification within a time limit, reduce the qualification of the enterprise or cancel the qualification of the enterprise according to the circumstances.