I have done this successfully, though I am not a success story myself, so I must accept that I can be seen as either a wise person dissuading people from stupid ideas, just as much as I can be seen as an idiot with no vision who would have told the beatles that the guitar is on the way out. This process takes a decent amount of emotional energy and probably isn't worth it in most cases.

Bring forward more enthusiasm for their ridiculous idea than they have, suggest concrete actions that they can take which will provide real feedback. They will either shrink from doing them (and be annoyed that you smile and ask them 'so have you ______ yet' without fail every single time they see you), or actually go and try it, hopefully failing early.

Here are some examples:

Guy has shitty movie idea he keeps pitching to everyone he knows (none of whom know anything about making movies), uses this to dominate conversations. I bought him a copy of 'Save the Cat' (didn't work), asked him what he was doing on a specific weekend ('nothing') and enthusiastically told him 'the (named) pitchfest is that weekend in burbank, plane tickets are $200 and the hotel is cheap AF, the whole trip costs less than I've seen you spend on stupid shit, you can really make this movie dream happen!!!!!!'

'thats an awesome app idea, let's get it working in an excel spreadsheet and see how it goes'

'thats an awesome product idea, grab a domain, put up a blank page, and spend $500(0 for the richer people) on google display network ads to drive traffic to an ad for the idea, see what your click numbers look like'

'oh yeah people would definitely pay for this art, throw up an ad on fiverr and see if you get any bites'

In every case, they either do it (rare, some people would rather have the identity as 'someone with ideas too good for the world' than having to actually risk failing at something and maybe losing that identity), or don't do it.

Three possible outcomes:

They do it, fail, and stop talking about it.

They don't do it, and stop talking about it because every time you bring it up, they get annoyed that they're being called out for being more talk than walk.

They do it and succeed, in which case, you were the person who believed in them and now a valued friend (also, you'll probably want them to keep talking about it, because the world has shown you that your model wasn't quite right)

I personally have some actually creative ideas (metric: can't find the idea expressed anywhere on the internet and experts in the relevant field say they have not seen it before), more 'almost' creative ideas (has been stated by a kook somewhere in the fringes), and a lot of misguided ideas (experts in the field have seen similar ideas many times from people new to the field) most are absolutely awful and none have made me rich. The ones directly related to my area of expertise are generally better than the ones which are not. The above advice for dealing with others mirrors the way I deal with my own ideas.

The ones I don't have the resources to test are available to anyone who cares to ask and possesses said resources btw.

I'm an undiscovered geniu...oh no.

The two most important things to understand about startup investing, as a business, are (1) that effectively all the returns are concentrated in a few big winners, and (2) that the best ideas look initially like bad ideas. ― Black Swan Farming by Paul Graham

I've known many people like you describe. One was an MBA who earns at least a quarter of a million dollars per year as a manager at a major tech company. Another is a former CEO. A third wrote software for a quantitative training firm. They all tried to start startups. They said lots of incorrect things to me. For example, the MBA once lectured me about how The Art of War is irrelevant to the study of business strategy.

Nothing I said ever convinced these confident people they were wrong. I quietly started my own startup. One after another, all of their businesses failed, often after wasting lots of money. Meanwhile, my startup chugs along.

Most of them learned their lesson and quit the startup game. Except the most capable one, who learned a different lesson and joined my startup as a co-founder.