Off-topic and not constructive questions are an annoyance. I would like to reformulate the remarks made in this unanswered question (and also partially this question) but bring hopefully more practical suggestions as to how to solve efficiently the problem the asker describes.
Finding the help/on-topic page (of each SE site) is way harder than it should be. It's not on the "path" of the newcomer when they want to ask a question for the first time and the existing mechanisms to prevent such bad questions are insufficient.
Here are two of those mechanisms that I've identified on a few SE sites and the corresponding suggestions. Please note that I don't know anything about how SE works and what may be customized on a per-site basis so maybe what follows is not generally applicable.
The tour. It doesn't insist nearly enough on the importance of questions being on-topic and, obviously, does not link to the help/on-topic page of the site. It states "Good answers are voted up and rise to the top". That should be followed with "Here's how to write a good question." (link is to the help center)
In the "Ask Question" page, there's the "How To Ask" section to the right. It says, for example on Stack Overflow but also on other sites, "Is your question about [insert site topic here]? We prefer questions that can be answered, not just discussed. Provide details. Share your research. If your question is about this website, ask it on meta instead." which, you may or may not agree, is not much in the way of pedagogy since it is vague and lacks direct links to the relevant, much more comprehensive guidelines of the Help Center. Suggestion: putting the short but efficient "Make sure your question is on-topic." (link is to the help/on-topic page) before the sentence about meta.
Suggestion in addition to the improvement of the above facilities:
- In the "Ask Question" page there is a list of already-asked questions that appears when the user starts typing in their question title. Have a banner appear along with that list, that clearly states "Here is the list of topics you may ask about. Off-topic questions will be closed." or something, maybe less abrupt, but just as short. The reason why this may be efficient is that the appearing animation already grabs the attention and that the perspective of the question being closed is not a pleasant one (I dislike the idea of threatening a user though, it's bad for the "mood" of the site). It would indeed be redundant with the How To Ask section, but not in a bad way I hope.
It's regrettable that the sites of a network of sites each characterized by their topic don't insist nearly enough on precisely their topic.