If a country bans it, it does not make any difference as other countries are not banning it simultaneously. Also, even if they completely and strictly ban it. Users can use various softwares to access the wallet. There is simply no way unless they take it as serious as child porn.
The only way to completely destroy Bitcoin is to ban internet. Even if all the countries in the world try to ban Bitcoin, it will exist as you need just a smartphone and internet connection to transact with Bitcoin. Banning a decentralized asset is never going to work. If the authorities are that adamant, then I would recommend a 51% attack. But that is going to take billions of USD in funds and lots of manpower.