On the how to write good answers page, the help center says
Answer well-asked questions
Not all questions can or should be answered here. Save yourself some frustration and avoid trying to answer questions which...
...are unclear or lacking specific details that can uniquely identify the problem.
...solicit opinions rather than facts.
...have already been asked and answered many times before.
...require too much guidance for you to answer in full, or request answers to multiple questions.
...are not about interpersonal skills as defined in the help center.
There are 5 reasons that a question can get closed, and all 5 of those reasons appear in this list as reasons that a question should not be answered.
So how do we moderate these answers?
With all of that in mind, we should moderate these answers the same way that we moderate other answers. If the answer is not helpful, downvote it. If it's not addressing the question, not backed up, or in some other way failing to meet our quality standards, flag it (if it's worth a flag it's also worth a downvote, so you should do that too). If you are flagging an answer, you should leave a comment.
For answers where the question has been edited
When the question has been edited from being a close-worthy question into one that should stay open, any answers addressing the original version need to be updated or deleted. You should use downvotes and flags on them as I've already mentioned. Additionally, you should leave a comment on the answer that explains that the original question was not a good fit for the site and has therefore been edited.
If you see an answer on a question that should be closed
Unfortunately, a lot of questions get answers even though they are off-topic, too broad, or opinion based. I have seen on a few occasions that someone will write a really good answer to an off-topic question that manages to address the on-topic parts of it. Most of the time though, answers to off-topic questions would not be good (specifically they wouldn't answer the question) if the question gets edited to be on topic. If you find one of these answers and the question hasn't been edited, you should downvote the answer and leave a comment explaining that the question is not a good fit, and therefore the answer will be invalidated if and when the question is fixed.