/diy/ - Do It Yourself

Got this radiator in my hallway that's being a dickhead; always on. The knob's kind of busted so I took it off, ziptied a metal shim to the pushpin and now it's "off".
Seems like the valve works fine enough but there's still heat coming off the pipes, not the radiator itself. Not much, anyhow.
I suspect the bottom right in/out is just flowing the entire time, and heat creeps up through the pipes since the main body is cool and the farthest corner cold.

I have 0 experience in plumbing/heating. I do have a fair amount of experience in general maintenance and DIY, as well as a fair number of general use tools. No pipe/plumbing specific tools beyond wrenches and the like.
Is there a way for me to fix this myself? If so could someone point me towards a guide/basic information document?
If not does anyone have a rough guesstimate on what a plumber would charge? Or perhaps how much the parts cost and time it'd take a professional.