On the subject of cancelled events:-
Bids are put in to the Airspace Utilisation Section of the Directorate of Airspace Policy at the Civil Aviation Authority (now you know why we use acronyms!) whose job it is to deconflict activity. If for example the RAF want to mark Betty's birthday with a flypast that involves assembling lots of heavy metal and routing it through someone else's aerobatic competition AUS will do the necessary to deconflict it and come up with a workable plan. They will then issue the NOTAM which will go over to AIS who'll load it into the European AIS database. The people who do this work normal office hours and while AIS do have staff on duty H24 it's not really practical to get a NOTAM cancelled if on the day the organisers take the decision to cancel due Wx (RIAT this year for example). This is becasue there's no-one on duty in AUS to issue the instruction to AIS.
WRT to bandits infringing notammed airspace. In the majority of cases the activity is the subject of a Nav Warning rather than a RA(T), conferring no rights on the organisers over the airspace concerned. It is therefore incumbent on the event organisers to make suitable safety arrangements. For example to have ground observers or a radar service watching for potential intruders and warning the participants of the threat.
Even in the case of a RA(T) the fact that you have rights over the airspace will not make you any less dead in the event of a collision so it's a good idea not to rely on the eyeballs of the display pilot alone. (IMHO)
Mike