If the misuse of the flag is caused by the fact the users didn't read the "it should possibly be an edit, a comment, another question, or deleted altogether" part, then what is probably confusing the users who are flagging is the "it does not answer the question" part, which seems to be interpreted as "it is the wrong answer."
Rephrasing the description as follows (or with similar words) could help.
This was posted as an answer, but it should possibly be an edit for the question, a comment, or another question.
The "or deleted altogether" is probably what causes the flag to be used from who want to flag an answer because they think it should be deleted. Removing that part, or rephrasing it could help too. As the only cases where an answer should be delete because it's not an answer are already listed, I would rather remove the "or deleted altogether" part.
In the case the users who flag an answer as not an answer don't read the description of the flag, then changing the description would not have any effect. In that case, there is few that can be done, if not educating the users on how to correctly use an SE site.