If governments worldwide declare Bitcoin illegal and enforce severe penalties, such as serious jail time for anyone caught using it in any way, people will stop using Bitcoin.
This is purely theoretical
While some might see it as a threat others might see it as an opportunity
So getting a unanimous decision to ban Bitcoin is highly unlikely especially with its popularity Now.
In theory Im thinking that all internet providers could effectively terminate anyone running or executing code relating to Bitcoin and then subsequently this would take it offline.
It is possible for powerful entities to make Bitcoin much harder to use and disrupt some part of the network
But completely shorting it down is extraordinarily difficult
Because of it's decentralized system, alternative method to communicate and it's current trend of global adoption.