A coordinated global effort to restrict or ban Bitcoin would indeed be monumental and likely impossible to enforce. The internet and cryptocurrency technologies are borderless, making it difficult for governments to control or restrict their use.
That will not be impossible. If governments clamp down on all exchanges which allows trade of Bitcoin and impose strict sanctions on anyone caught trading P2P it will reduce the use of Bitcoin significantly. There will be a few channels, which will be termed "black markets" that still allow trades, but not many will want to go down that route as it will be a crime at that time
An isolated restriction by one government will be difficult to enforce but a coordinated approach, which is highly unlikely to happen, will be more successful.
That's true because in as much as it seems that the government do not have a power over Bitcoin but at some point they could possibly have an influence to the users within there jurisdiction because after restricting all the exchanges they believe there citizens have access on in terms of dealing with Bitcoin transaction the population on Bitcoin will be reduced because it has created a limitation from any possible way people can easily reach out to Bitcoin and besides like you said if a sanction is given that whomever they caught dealing on any transaction will be jailed, the fear alone will certainly discouraged so many people so actually government cannot control Bitcoin but they can have a serious influence within those on there jurisdiction but the good thing is that even if a particular government imposes such thing, other countries will continue there Bitcoin investment.