Assuming it is a common looped system, then you cannot control one room individually without affecting other rooms also. If it is copper baseboard, you can tape the opening at the top or bottom of the baseboard to prevent or limit air flow through it. Since they use the air flow for most of their heating effect, stopping or reducing it will moderate the heat outpu.
If limiting the air flow does not work, I might try removing some of the aluminum heating fins and putting on pipe insulation a small section at a time. Don’t toss the fins in case you want to re-install them.