I agree that abusive language should not be used and the observer should not be coerced into changing a score.
The AMCA do have a comprehensive rule book which includes Rider Conduct. The first offence for which a suspension from the meeting can be made states. Any act which is of a violent, threatening or abusive nature to any official rider, passenger or any other person.
As said in other posts. Trials should be fun. We don't need these people.