If no-one else has responded, explain to them what they are getting wrong and help them to fix it (your option 3). If someone else is already doing that, there’s nothing for you to do.
If your help turns into a longer discussion, take it to direct messages, so that you don’t have a mentoring session going on in the public forum. Conversely, if you’re not willing to mentor the poster, just say nothing and let someone else do so. Or if you prefer, say something along the lines of “Sorry, your idea needs more development to be useful here - I’m not able to offer you any advice, but hopefully someone else will (or you may be able to get a better sense of what’s needed by reviewing other posts in this category to see what a successful idea should look like)”.
Posters are looking for human interaction. We may not always get everything right, but if we’re not here to interact with other people, what’s the point? Nobody’s expecting perfection, just a willingness to help. (Mentoring is hard and it’s a lot of effort. But we either want people to get involved, or we don’t. In some ways it’s as simple as that).
Because no-one likes getting an automated response.
Thank you for that comment. It’s genuinely appreciated.
Basically, my problem with it is that it’s becoming a forum for people who want things from Python, as opposed to people who want to contribute to Python. While I appreciate that not everyone has the expertise to contribute (for example) C code to implement a highly optimised data structure into Python’s stdlib, I do expect people to participate with an attitude of “if I’m not willing to do the work myself, what right to I have to expect anyone else to do it for me?” And it’s not necessarily anything specific that I want them to do. It’s just an approach - if someone says “this has come up before” then go and look rather than saying “can you give me a link?” (if they could do so without effort, they would have!)
Often people post to the ideas category without even properly understanding how Python works. Ideas along the lines of “all iterators should have a map method” show that the poster doesn’t know that iterator is a protocol rather than a base class - and quite possibly doesn’t even understand what a protocol, of duck typing, is. Many ideas that are of the form “language X does this, why can’t Python?” (whether that’s explicit or implied) demonstrate this lack of willingness to even learn how Python works.
For me, it’s important to remember that the ideas category is for doing language design. If you don’t know anything about language design, and about how Python in particular works “under the hood” (in the broadest sense, at least) then maybe go away and find out before joining in? Or maybe stay, but read the ongoing discussions before starting to post yourself. Too few people bother to lurk and get a sense of the community before barging it. Hence low-quality “drive by” proposals 
To give a somewhat silly example, I’d be quite happy to sit in the pub with some friends and discuss “wouldn’t it be nice if my car could go on water like a hovercraft?” But I’d never even think of going into my local garage and suggesting to the mechanics there that my car should go on water like a hovercraft. IMO, the ideas category is the garage, and the help category is more like the pub.
The last thing I want is to try to claim any sort of authority simply by virtue of being a core dev, or having been around a long time, but at the same time, I get very frustrated when people come into the ideas category and make proposals or claims without taking any account of the history, or showing any signs that they are willing to listen to people who clearly have more experience than they do (the mere fact of saying “this has come up before” is evidence that “I’ve been around long enough to have seen previous versions of this proposal”).
I hope that’s of some use. It was a bit of a brain dump, as I don’t have much spare time this evening.