They can try, but eventually people who don't want to be under the thumb of the control-freak authoritarians around the world (socialist, fascist, communist, whatever the stripe) will just remain in bitcoin and not have to deal with them. 

I would also add that it seems unlikely that "all" governments will agree to ban bitcoin.  Some will see the benefit to not doing so and competition will force others to follow.

Think about the systems or political parties in the countries/states/regions trying to (in some cases repeatedly) ban bitcoin:  China.  New York.  India.  All want to control people and don't trust people to make decisions for themselves. 

Which side of history do people want to be on?  The one advocating for freedom and liberty, or the one advocating for statism, authoritarianism, totalitarianism, and control?