Countries can banned BTC/Cryptocurrency but they can't stop it. I believe there are still lot of people doing business in cryptocurrency even though the government banned those things.
Yes that is right government can ban bitcoin only in his country. The countries in which Bitcoin is ban is china, Bangladesh, Bolivia, Malaysia, Ecuador. Every country has their own agenda behind Bitcoin ban. Chinese government ban it because they doesn't want to loose their country currency value in their own country . And the government doesn't want digital currency like bitcoin affects their banking system. Bolivia, Malaysia and Ecuador government ban it because it directly affect their country financial growth and economy value and it is also banned in some country because it associated with many criminal activities. 