Why do you want to prevent that? Is it a problem to add more than one trust level group? I know that usually it’s not needed, but I like that I can add an additional, lower trust level to a setting, because then I can remove it later without having to remember which TL group was in there before.
I think it is way more important to understand the group concept for category permissions, because otherwise you could think that creating a category for the trust level 0 group will only be visible to new users. That is definitely a problem.