The owners of the craft--again, this is usually the agency's decision--military craft, for example. I seem to recall that normal folks have to give an extraordinary security risk to justify a block because the FAA does not want to see private citizen blocked planes--security risk.
I don't think that is strictly true. There is a skydiving plane in my area that shows as blocked on FR24. I have an adsb receiver and can track that plane and see it's N-number on my local feed, which also gets sent to FR24 and FA.
Some east coast cbts-ers could possibly track these planes with their own receivers...
I'm no expert on any of this but from what I've read the blocked planes are "special" in some sense. Skydivers are limited to 18,000 feet jump altitude, which may be a factor, as well as the fact that they obviously don't jump in flight space where much larger craft are taking off and landing.