This is that post:
I would approach this the Socratic way, by asking questions to help the person realise by understanding rather than just trying to impart your knowledge directly to them.
Is fiat money a scam? (of course you expect to hear a resounding 'no')
Is it a scam that governments and banks can print as much money as they want?
Is it a scam that because of that, fiat money is worth less in the future than in the past, eroding the value your savings which you worked hard for?
Do you really think that people, when given the power to print money, will not abuse it?
Who regulates the printing of money? Do they do a good job of it?
Is it a scam that there are a few powerful people pulling the levers of the economy?
Are you happy with a few people telling you what is valuable and what is not? Because that's essentially what fiat money leads to.
What are the benefits of gold as money?
All those benefits are why bitcoin is also good compared to fiat money.
What drawbacks are there to using gold?
Bitcoin has solved those problems also.
Are shares a scam?
What about if you bought apple or google shares when they just started up, is that a scam? Why not?
Are shares in [valuable but socially/environmentally damaging company] a scam?
These are exactly the same reasons bitcoin is not a scam.
Is paying with cash a scam?
What is good about paying with cash?
All the reasons you like cash are true of bitcoin also.
What is bad about paying with cash?
Bitcoin has solved this as well.
Do you know about (any of the more advanced economic things such as options, futures, derivatives)?
How many people in the general population understand these concepts? (not many)
Does that make it a scam? (no)
Why not? (they could find out if they wanted, the detail doesn't need to be known about by most people... etc)
Same is true of bitcoin but moreso, because the basis of it, the code, has been reviewed by experts and validated as not being a scam - if it were a scam it would be apparent in the code immediately and nobody would support it at all.
Do you ever get spam from Nigeria?
If it said [the name of the bank of the person you are talking to] in that email, would you say [their bank] is a scam?
The same is true of the services surrounding bitcoin - just because a third party is untrustworthy, it does not mean the core concept is broken, just that the services surrounding it needed to improve, which they have.
Ultimately the value of something reflects the value of it to society. Or rather, it SHOULD do that. Fiat money greatly clouds these issues because it funds unpopular wars, undesirable corporations, and excessive bureaucracy. Bitcoin is not a scam because it allows the true value of goods and services to be reflected in their price.
Of course if you don't know your shit you can get your arse handed to you, and even if you do know your shit you will be into a rollicking argument very quickly, but these are all interesting questions that should at least begin to impart doubt into the minds of people who think bitcoin is a scam. At the very least it will hopefully make them realise it's no LESS of a scam than any/most other monetary systems.