I flagged this post as 'Not Constructive' but the OP is protesting. What should I do?
There are other people who have agreed with you and hence the post is closed now. You need not worry if the OP protests. If he is right, then he can vote to re-open and wait for other people in community to support him.
However, if OP is protesting, I dont think you can do anything better than explaining him why the post if not constructive. If OP is not convinced and you know that he is wrong, then you can just ignore and carry on to help other posts.