How did you check the monovalve?
I ask you this because there is no way hot water comes inside (really suctioned through) the heater radiator if the monovalve recieves the "close" signal (12V) from the climate control and it works ok. All this, assuming all the hoses are well connected and no one is bypassing the MV.
Thinking about damages after blocking the heater radiator, the answer is no. Blocking the waterflow is exactly what the MV does when it is closed. Maybe you will accelerate the wear of the heater pump due the extra effort to suction hot water (coolant) with no results . Could be a good idea disconnect it. But i think too you can solve your heater problem before doing all this. Let me know.