I asked a question that recently received this answer. As stated in the comment, this is clearly some sort of fan-fiction. Aside from the obvious downvote, which (if any) flag would be appropriate here? I feel "Not an Answer" isn't any good, because they are trying (and failing) to provide an answer, yet I feel "Spam" isn't sufficient either.
The stated policy is that "flags should not be used to indicate technical inaccuracies, or an altogether wrong answer".
That said... There's a case to be made that the answer should be deleted on the grounds that "it is very low quality" (or even just flagged as "other" with an explanation why it needs closing) but frankly, I'm happy to just leave it for now. The community is doing its job in downvoting it to hell and there's a comment explaining to the poster why.
UPDATE : In light of receiving several community flags (and reflecting a substantial numbers of downvotes) I've decided to delete the answer in question, if for no better reason than for the sake of the poster.
Based on discussion here, the consensus was that we don't assume an implicit requirement of canon answers. So, unless the question does specify that canon answers are sought, non-canon answers are technically fair game.
That said, if you find it to be less than useful, you are more than welcome to downvote it.
*Note: Yes, the other discussion was largely focused on canon versus speculation-based answers, but the original question was a bit broader.