You need to be very careful in this situation. Determining what is careless and wreckles is not clearly defined and is very subjective. Acting as a CD for the AMA at a sanctioned event you are making that determination. Your opinion and how you handle it are going to come under fire if you don't handle it well. My question to you is whether you believe the flying was within the safe flying boundaries established, under positive control, within the reasonably tested limitations of the aircraft structurally, within your established rules for the field and at the pilot meeting and was not done with an intent of causing harm or putting anyone in immediate danger? If so, I would not consider that wreckless or careless and I agree with the pilot. You should have made your expectation clear in the pilots meeting if you did not want planes approaching toward the safety line. There is no official rule against this and again is very subjective. At that point you may be putting unfair restrictions on the other flyers though. Are you going to tell 3d flyers they cant approach the flight line inverted at a walking pace and push up to a hover. That popular manuever is here to stay and is very safe. Clearly from your statement you had a previous opinion of this person and did not like their flying style. It is not fair to place such prejudice on a fellow flyer while acting as a CD. Now if he or she was obviously, intentionally and consistently wreckless during the event, it is appropriate to speak up and you were responsible in doing so. How you do it is important. People don't like to be touched while they are flying (which is usually more dangerous than the incident you are trying to prevent), told they are wrong, ridiculed for something unintentional or chastised for what you let your buddy do all the time. You have to use care in how you approach such challenges. I prefer the technique of self blame. Basically I would privately say, "hey, I should have been more clear. I wanted to cover in the pilots' meeting that because there are so many spectators here and we are pushing the limits of the airspace today, I was thinking we should all avoid flying torward the flight line at high speed. Are you okay with that? I'm just going around letting everyone know. Sorry man." As an additional or other option, you can make an announcement to everyone saying you forgot to cover it. This is just one example of many ways of softening the blow. No one likes to be told what to do out there. There is nothing worse than an AMA bully, especially one making up rules. Being CD isn't for everyone. I haven't done it yet, but I've seen good and bad CDs. It's a tough job. I'm sorry you had a bad experience. I blame a lot of that on the unchecked cultural problems that exist by people self deputizing themselves as uninformed AMA Police and enforcing outdated or non-existent rules. Another major issue is aggressively policing rules that are inadvertently or unintentionally broke, which is the easiest way to irritate someone at the field. There's nothing like rubbing salt in the wound of embarrassment to drive members crazy, or away all together.