...tell them you didn't ask for their opinion.
That's the whole point of Bitcoin. That's what we mean when we say it's 'fuck you money'.
Some people will only learn once it gets 'banned' then people carry on using it anyway.
I prefer to go with: "Why don't you just ban sugar while you're at it?"
It's almost impossible for them to avoid a conversation about how decentralized bitcoin mining is and jurisdictional arbitrage at that point.
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If someone wants to ban my bitcoin, my response is "say 'please' and I'll consider it" :)
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My response to those people is to say:
"Good luck."
Then, watch their face contort as they try to understand what that means. lol