Yes, you can shut off individual radiators in both 1 pipe and 2 pipe systems. 2 pipe systems you can also "throttle" the valves, meaning partially close them to try to get a little less heat out of that particular radiator. 1 pipe, you must have valve either fully open or fully closed.
How much of the building are you thinking of shutting down? How cold will it get in there? Is there plumbing lines, etc that could freeze up and burst if allowed to get too cold? Plaster will generally start cracking if temp goes below about 50 degrees consistently, if that's a concern. There's a lot of factors, more info about your situation would help.
If you can get the load that you're supplying down significantly, you may be able to shut down one of your boilers. Again, more info would be needed to determine that, but that could really help.
Then its just a matter of making your system efficient - insulate steam mains, run at low pressure, vent system well, etc. Combining all these things on an old system that has been messed with by a bunch of guys who don't know what they're doing on steam can often save 1/4 to 1/3 of operating costs w/o reducing heat output at all. Reducing load on system (by shutting down areas) can reduce that even further.