Indeed, knives can be used for anything other than being kitchen inventory, including a murder weapon. But you can't ban knives because their home use (other than homicide, obviously) by far exceeds their other uses. Thus it is out of the question as entirely impractical. Further, some varieties are actually banned from possession unless you have a license (e.g. hunting knives and anything looking more like a sword than a knife). On the hand, banning Bitcoin will not have a lot of side effects and unintended consequences apart from a bunch of nerds wining and crying over here and over there. Simply put, banning Bitcoin is not a big deal
Bitcoin has a lot of benefits like transferring money from one country to another in blink of second without consuming any extra third party fee etc. Banning bitcoin will have a serious impact on economy and whats more better is that due to decentralization, no one can completely ban bitcoin and people will still be able to find the ways to use it
I'm no longer sure about the blink of a second
Nowadays, it is actually more like hours than seconds (let alone a couple blinks). Third party fees are also irrelevant here as miners are that third party anyway, so under these circumstances it is the size of the fee which ultimately counts. But in this department Bitcoin doesn't shine either. Regarding Bitcoin's effect on the economy, it is greatly exaggerated, and if the role of a devil's advocate should be played to perfection, this effect exists only in the minds (read, imagination) of Bitcoin holders. As you can see, banning Bitcoin in its entirety will turn out mostly unnoticed by the wider public