Hi.
I was at a car sporting event on the weekend. Where the organiser had fenced off the area where the car race was being held and charged the public an entrance fee. This area does have yellow drone assist areas with cautions which have always been in place, saying fly with caution and may attract attention. I did not have my drone, so did not have any plans to fly. But did think that you could get some great footage from the event and maybe something I could do next year, when the event was held again.
While there was a break in the event an announcement came over the speakers system giving instructions and information. One of the announcements, said that being a live sport event that no drone flying was allowed, and the previous day, someone was flying a drone and they stopped the event until the drone pilot had stopped his flying.
My question is can they actually stop you from flying a drone in that airspace, which would normally be allowed, but with cautions, when the sporting event is not being held.
Not unless they apply for a temporary restiction with the CAA and you are flying within the regulations. I.E. not over crowds etc. etc.
But how selfish would the pilot have to be to cause an event to stop until he did, spoiling it for everyone else
Depends where the drone was actually flying. For all we know it could have been flying around the periphery of the event and have possibly been of very little consequence. The organisers could have been a bit heavy handed in their approach and stopping the event. There again, the drone could have been flying directly over the event and any stoppage could have been justified. 
Is that because there was betting on the event?
I guess there is a risk that someone might live stream the event and change the bets being placed??
If the event organiser doesn’t want me to fly my drone then it stays in the boot of the car. No point in winding up the organiser and everyone else attending, it isn’t going to be a pleasant relaxing enjoyable flight.
Pretty much what @stevesb said.
Sometimes as a drone flyer we have to give a thought to our fellow human beings. Even though we might be flying completely legally it doesn’t really mean we should be. I try to put myself in other peoples shoes and think “would I get really upset if the roles were reversed?” Having an event such as this stopped, while not illegal, is not something I would do.
There should be a very simple rule added to the CAA drone code
“Don’t Be A Dick”
(I’m not saying this is you, by the way. Just something I have to tell myself on occasion)
This should be the only rule. 
It’s all the drone code should have been
If you really want to capture some footage of the event without upsetting the organizer, hold the drone in your hand, walk to a good vantage point, turn your camera on. You can turn to follow a car passing, use zoom (if your drone has that feature), tilt the camera up or down, and the gimbal will smooth out all your movements.
Sandown? Is the yellow zone for the airports?