When removing a spud from a steam radiator, it can happen that the bushing in radiator gets damaged. The faster you try to get the brass spud out, the easier it is to damage the thread in the cast iron bushing.
Luckily the bushings can be replaced. Just make sure you get the right size, and use the largest pipe wrench you have or can afford.
The larger the screws you try to remove the larger the tool you need to use. Muscle will be your friend in this case.
Search Google for "MatcoNorca Black Radiator Bushing" to find them (just an example, you may find other brands too).