Did your radiator have one or two pipes? If it is a two pipe system, I would think twice about capping it off. It all depends on how it is piped. If they put the radiators in a series loop, you would stop the rest of the system from getting hot water. Now if they plumbed them in a parallel type as pictured in the bottom part of the picture you might be able to get away with it. But there is a chance of causing an unbalanced load.