So now, the question is closed which means I cannot respond.
That's just not true. You can still comment and vote on closed questions, allowing you to respond. The only thing closing prevents you from doing is posting a competing answer.
Note that when a question is closed current answers aren't deleted. Even if the question is not constructive and there is a low quality answer (which is of low quality for all of the reasons the questions was closed, namely that it's not constructive) that answer will not be deleted. As long as the answer is making an attempt to answer the question, and doesn't contain insulting, abusive, or spam content it won't be deleted, as you saw when your flag was declined.
In retrospect it looks as if it would have been better to answer the question, even if this invites even more non-constructive behaviour. So what would have been the right way to deal with this?
When you notice a problem, the first thing you should do is make sure you're not contributing to it. Rather than saying, "So and so is posting unconstructive answers, I'll do the same and try to beat the closure." you should first ensure that you're not posting answers to questions you feel are not constructive (or that you know the SO community feels are not constructive).
As for what proactive actions you can take, you can:
- Downvote such questions, if you feel it's warrented.
- Vote to close such questions, or flag them, so that they are closed more quickly.
- Comment on such questions/answers in an attempt to help inform those asking and answering why these types of questions are inappropriate for this site.
- In more extreme cases, once the question is closed, it may end up being deleted (taking the answers with it). If you feel a question is of extremely low value, or it is actively harmful for it to remain, even in a closed state, the question may be eventually deleted. You can vote/flag to delete if you feel that's the appropriate action. (I don't think this particular question warrants deletion, just closure, as it's value is not negative as it stands.)