Countries can outlaw cryptocurrencies by banning the use or purchase of crypto. However, this is incredibly difficult to enforce. Several nations have banned Bitcoin, however, people are using VPNs to access exchanges, and store their wealth either on or offline.
A decentralized currency will last forever. The software and protocol will always exist. Whether people use it or not is another question. And whether people use it has to do with the general ecosystem behind crypto currency and whether the government will step in to regulate the industry. It doesn't take much for people to divest from Bitcoin if the government were to come in and threaten high taxes on crypto assets. Alternatively, a symbolic ban would be enough to drive users of crypto away, even if there isn't any actual enforcement.