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New users posting thanks, comments or new questions as answers on existing questions is very common. It is also something that should be easily handled by the community itself, without requiring diamond moderators to intervene.
The action that need to be taken in such a case are:
- commenting on the post to explain that answers shouldn't be used in this way
- deleting the post
- in rare cases where the post contains valuable information, but isn't an answer, it can be converted to a comment
Problems with the current tools
Commenting can be done by the community, though the behaviour of notifications on deleted posts needs to be changed like I requested in my previous feature request to enable the community to effectively handle this.
In theory, the community can delete such posts. The post needs to be downvoted and three 20k+ users need to vote to delete it. There are several problems with that, making community deletion rather rare.
The other problem with community deletion is that the bar is very high, there just aren't that many 20k+ users on most site. And reputation alone isn't a very good way to determine who should have access to that kind of moderator tool.
Conversion to comment is a mod-only feature, and probably should stay that way. The temptation to do the nicer thing and just convert to a comment when the post should probably be deleted is pretty high.
My possible solution
There are a lot of users that know the SE system well and can reliably identify non-answers, but don't have the 20k reputation necessary to fully act on them. It shouldn't be too hard to identify these user based on their flagging volume and success. I think that a criterium like "flagged x posts as not an answer with at least y% declared valid" would be a far better way to identify trustworthy users than the current 20k+ reputation requirement.
For those users that are determined to be trustworthy, either the "not an answer" flags should automatically delete the post when enough of them are cast by trusted users and if at least one of them has commented on the post. This would enable those users to deal with "not an answer" flags without requiring a moderator to step in.
As a way to safeguard these expanded privileges against abuse or mistakes I'd suggest that even a single "disputed" flag would disable the automatic mechanisms and force the flag to be handled by a diamond moderator.
A different possibility would be to just give those users a delete link on "not an answer"-flagged posts.