There have been a number of well-written responses to this question, and I'd like to offer comment on them, as well as the original question.
If someone posts an answer that is essentially "In my opinion,...", should I flag the answer, or just downvote it and move on? And if I do flag it, should it be "not an answer", "very low quality", or a custom flag?
To answer the original thrust of the question, if the answer is entirely an opinion and is not the legally correct answer then it should at least be downvoted. Of course, this introduces a bit of a chicken-and-egg problem for users who are unable to discern the "correct" answer. Fortunately, I believe we have an engaged user base who are able to do so.
In the event that the answer happens to be correct while appearing to be an opinion, comments encouraging the provision of sources should be added, or even a comment adding the reference which would support the answer.
In the event that the answer truly is an opinion with no legal merit, the answer should be treated as low-quality, downvoted, and if it does not attempt to answer the question, flagged.
Dale's answer refers to a problem which we have had since the site's inception: we do not have a way to distinguish expert users from non-expert users. Fortunately, in my opinion, that is not necessary. As also pointed out, upvotes and downvotes from users on the site should be used to distinguish the correct, well-written answers from the incorrect, well-written answers.
user6726's answer alludes to the problem I mentioned above: that there are two types of opinion answers: one which is legally correct but doesn't cite any sources to support this, and the other is just a statement normative declaration. Again, here, the problem of distinguishing these answers should be taken care of by voting and flagging as appropriate.
Iñaki Viggers's answer seems to mischaracterise the purpose of the site. I cannot say it clearly enough: this is not a site for personal opinions. It is a site for questions and answers about the law. There are any number of sites which are suited for discussing opinions about the law; this is not one of them. I suggest looking for a social media platform on which to voice such opinions.
While answers should not be flagged just because they are wrong, I would almost go so far as to say that if they are purely opinion and wrong, they should be flagged.
I don't even know what "unsubstantiated" means with regards to voting on this site, nor how one would come to that conclusion, given the votes are also anonymous? I favour writing a comment, but in some cases, a comment is not needed, or has already been made; in that case a vote on the comment would also be helpful.
Marcus Potter's answer is pretty much on point, except that answers that don't attempt to answer the question should probably be flagged. Moderators do not look at how often flags happen, we just look at the post and consider whether it should be removed or edited. A "boy who cried wolf" scenario will not happen.