Fill a hole around a radiator pipe
That's a large hole I would probably purchase a split flange cover (eschutcheon) 


that goes around the pipe but unless you can find a large one a patch job will be in order first. The split flanges you want will be metal as the heat may cause problems for a plastic one. On the few hot water /steam systems I have worked on they usually have a small gap and fill the gap with insulation then a cover if you seal it tight you would be amazed how much noise it can make.