You are assuming this does not already happen?
In these kinds of situations, the moderator will bring up the issue with fellow moderators and the administrators and a resolution of the problem is found that way. And it will usually be up to the moderator to implement in his/her forum. Sometimes the administrator will step in as well and stop the member (by warning or implementing a ban) who has been causing the issues in the sub-forum.
There seems to be this perceived belief that moderators do not discuss the issues with the rest of the moderators and the administration. Nothing could be further from the truth. We do discuss the issues and we do discuss the members who have a problem with us as individuals and how we moderate.
Even if it is done in the manner you are suggesting, the member could then turn around and accuse the moderator of lying.. which, believe it or not, does happen.. I suspect they do not realise that the administrators of this site can view deleted posts and threads to see exactly what has happened to make up their own minds.
What has happened in Sam's situation is that the members in question are complaining about her moderation when she posts in forums that are outside of her sub-forums... where she is posting as a member and not as a moderator. When these members then decide to go into the sub-forums where she moderates and post off-topic posts and she deletes them and/or warns them about it, they then accuse her of abusing her powers.. which she is not doing. She is acting well within her power to keep the threads on topic and deleting off-topic posts, insults, flames and trolling. To accuse her of abusing her powers in such instances is a way to try to get revenge because they don't like what she posts as a member in other sub-forums.
Your proposal would be removing her moderator powers when it came to such members when they take it upon themselves to flame, insult, troll and post off-topic in her sub-forum.. And that would be unfair to the members who are not causing any issues and want her to moderate and keep the threads on-topic. She has an obligation to those members to ensure that the posts they are diligently posting in or have started in her sub-forum and by stating she should somehow pass the buck to the administrators of this site or to other moderators when it comes to the problem posters is unfair to her and most importantly, it is unfair to the members who are not breaking any rules.