How do I permanently remove a steam radiator?
If you have permission to remove the radiator:

1) Make sure the system will not be actively producing steam. 

2) Use a large open-ended or adjustable wrench to loosen the union between the brass radiator valve and the radiator. It will likely be difficult and you may need to tap on the union nut with a hammer or use a section of pipe to extend the leverage of your wrench to loosen it.  The radiator is disconnected.  

3) Use an open ended wrench and a pipe wrench to remove the brass valve.  Spin the valve off with the open ended wrench while providing counter-pressureon the elbow with the pipe wrench.  Only using one wrench may stress and crack the fittings. 

4) Plug the pipe. Depending on which fitting you choose to remove, you may want a square headed pipe plug (if you have exposed female threads) or a pipe cap (if you have exposed male threads.   Based on the photo, it looks like either a 1" or 1.25" pipe.   

If you do not have permission to remove the radiator, but want to stop the hissing and heat... This is one-pipe steam, and either of the following options will solve two of your problems.

1) Turn your silver Hoffman valve upside down.  This prevents the vent from venting.   

2) Remove the silver Hoffman vent and replace it with a 1/8" pipe plug with a square head.