The good thing about this is that they are posted as answers, and not comments. Therefore, they can be voted on, edited, and commented on specifically without creating comment noise elsewhere.
If the author intended to say it was just an untested suggestion, and you're reasonably sure of this, edit the post to include this information.
If such an edit would go against the intent of the author, you should usually not do it; instead, comment. Also consider downvoting.
- While commenting is valuable in these instances, downvoting is still valuable in and of itself. If, for whatever reason, you decide not to leave a comment, please still downvote if the post merits it.
If you're able and willing to expand it to a full, good quality-answer, then of course do that. You are under no obligation to do so, however. Furthermore:
- If your answer is mostly new, you should feel free to post it separately.
- Often, an answer that gets it right is a very effective way to show (usually without trying) that another answer is wrong or dangerous.
- I emphasize again however that downvoting bad posts is inherently good and not at all incompatible with posting your own answer. (You shouldn't indiscriminately downvote all "competing" answers, of course. But if an answer is worth downvoting before you posted yours, it is usually still worth downvoting.)
If the post is only marginally even a suggestion, could not be followed at all, or can only be even a little bit helpful to someone who already knows how to solve the problem, and you cannot fix it (or it'd be inappropriate to do so because the author's intentions are clear), then you should flag the post as not an answer or (less frequently applicable) very low quality. If you think it might not be clear why this post does not answer the question at all, you should consider using a custom flag instead of the NAA or VLQ flags. This way you can explain the reason.
- I don't downvote all or even most posts I flag, and I don't really recommend doing so. But flagging a post doesn't mean you can't or shouldn't downvote it. If the post is deleted, both its author and you will regain the lost reputation from the downvote.
If the post answers something that is decidedly not the question that was asked, flag it as not an answer. If it's unclear, comment (or custom flag).
If the post seems like a suggestion at first glance but is actually spam, which sometimes happens, flag it as spam. If it's not immediately obvious it's spam, you can also custom flag it.
- If you're not sure if it's spam but pretty sure it should be removed, you should custom flag it to explain that.
- Sometimes you can figure out of someone is a spammer but looking at their other questions and answers, as well as their edit suggestions. Often spammers have multiple accounts across the SE network, so it's worth checking all of them. This can provide useful information to distinguish between some product's fanboy/fangirl offering it semi-inappropriately as a solution to someone's peripherally related problem (still VLQ or NAA most of the time), versus actual, full-blown spam.