A. Who should make that threat? Who has the authority to actually starve / let them go without medication and say that on behalf of everybody?
B. What to do if they do not bow down to the ultimatum?
C. I don't find it overly likely that Russia will cooperate.
D. Russia have stated that their conditions for peace includes them keeping what they have stolen.
E. It would be highly unethical to follow through on the threat and damaging for morale if we don't.
F. Putin doesn't really care much about russian lives, he would sacrifice them in order to please himself.

We can't be monsters and we would be shit at pretending to be that. In the longer term it would risk that our local fools try the same logic on states like Hungary, Poland, perhaps UK. Accepting threats as a solution is easy for bullies.