I don't know how hard it is, but the ISS uses big radiators to dump excess heat into space.
> The Station's outstretched radiators are made of honeycomb aluminum panels. There are 14 panels, each measuring 6 by 10 feet (1.8 by 3 meters), for a total of 1680 square feet (156 square meters) of ammonia-tubing-filled heat exchange area.
There's hard theoretical limits on the rate at which you radiate heat, so without contact with an external medium or dumping mass, you're pretty much stuck with limits based on how big your radiators are