Not constructive is one of the flag reasons shown to you as an option, when you click the flag icon next to a comment.
So yes, unconstructive comments should be flagged.
Find the flag icon by moving your mouse pointer directly to the left of the comment. If you're using a tablet device, tap to the left of the comment. Then click the flag icon.
Clicking the flag icon brings up a dialog where you choose your flag reason.
But wrong doesn't always mean unconstructive. Similarly, correct comments are not always constructive.
Comments are "second class citizens" in the Stack Exchange system. It is rare that you should be reluctant to flag a comment that seems unconstructive, and I have no reason to think this situations in in any way exceptional. Furthermore, Luis Alvarado removed the comments, so I cannot see them, but that he removed them suggests that they really were unconstructive. Still, you should not flag every comment you see that is wrong. Sometimes a comment is wrong but constructive in the sense that it is more helpful to respond to it, or it's already been responded to such that removing it would make other comments hard to understand, or it serves the purpose of clarifying what someone thinks even if others disagree (in this situation, the comment is constructive if what they think is relevant to one or more posts to which the comment pertains).
Answers should be posted as answers. It's sometimes handy to tell people that. But if it wouldn't add much value as an answer and is not constructive as a comment, then flagging it as not constructive is the way to go.