You cant really ban it, but illegality could affect the ability to convert it into fiat currency.  This would have an adverse effect on its ability to be used as a medium of exchange and therefore its value versus the dollar, with a flow on effect to miners and other areas. 

Bitcoin would have to be illegal in all major economies, if not the world for this to take effect though. If you had even one western nation with a reasonable population where it could freely be exchanged for the dollar enterprising individuals would find a way to profit from facilitating trade for the rest of us. 

Banning bitcoin is only practical because the majority of the world doesn't understand bitcoin or what it can offer them.  This probably also why many governments haven't bothered with doing it.  If they were to decide it was a serious issue sometime in the future I think the cat would already be out of the bag. 

I could see this happening in the future with the introduction of private blockchains and government controlled cryptocurrencies.  Once more people understand the concept of digital currencies its a natural jump to have a demand for something like bitcoin that circumvents the controls that a state or privately controlled cryptocurrency will have. When people recognize that the value of bitcoin is not in its valuation in fiat but as a unit of trade that is uncontrollable by any entity then it will be impossible to ban.