> Well the decentralised nature of most crypto currencies prevent this sort of ban...
Just because Bitcoin is decentralized doesn't mean that a country can't ban it within its own borders. To do that, all a country would need to do is pass a law making it a crime to own or use Bitcoin, while also prosecuting people who didn't report profits from illicit Bitcoin transactions with tax evasion. At that point, no legitimate business in the country would be able to accept Bitcoin or convert it to cash, and the vast majority of citizens wouldn't risk fines or imprisonment to use it.
Since, at this time, a very small fraction of the population uses crytocurrencies (or even knows what they are), enacting such a law probably wouldn't face a lot of resistance, especially if it were presented as a measure to fight crime or terrorism.
Just because Bitcoin is decentralized doesn't mean that a country can't ban it within its own borders. To do that, all a country would need to do is pass a law making it a crime to own or use Bitcoin, while also prosecuting people who didn't report profits from illicit Bitcoin transactions with tax evasion. At that point, no legitimate business in the country would be able to accept Bitcoin or convert it to cash, and the vast majority of citizens wouldn't risk fines or imprisonment to use it.
Since, at this time, a very small fraction of the population uses crytocurrencies (or even knows what they are), enacting such a law probably wouldn't face a lot of resistance, especially if it were presented as a measure to fight crime or terrorism.