Yes, they should ban people who are not breaking any rules. They need to accept pvp is going to happen, and make that part of the fun of the event, or just forget it.
Actually, purposely attending an EM Event for the sake of griefing said event is, has been, and forever will be a violation of the rules of conduct, specifically item 24, which reads:
You will not do anything else that interferes with the ability of other Ultima Online users to enjoy playing the game in accordance with its rules, or that increases the expense or difficulty of Electronic Arts in maintaining the Ultima Online service for the enjoyment of all its users.
When griefers specifically target a scheduled event with the sole purpose of degrading said event into something that the EMs themselves can do pretty much nothing about, it is interfering with BioWare Mythic's ability to provide part of their service to player.
And really, the idea that people should just accept griefing is a bunch of bologna. People showing up to turn these events into the hellfests that they become are not doing so because they want to play, or because they want to take part in the event, they're doing it simply because they know there will be a number of "victims" present that they can exact their idea of "play" onto.
It's this ludicrous mentality that created Trammel in the first place.