Bitcoin cannot be made illegal, it is decentralized which means no one party can control it.
You would have to go into every miners house and remove their mining equipment to stop Bitcoin.
No, not really. There are some ways to block bitcoin quite efficiently. While bitcoin has no central authority that is not a problem for governments to take action against it.
We have seen already tests of mass blocking every bitcoin service in some countries, Venezuela conducted that tests recently and everybody was in shock.
Bitcoin p2p trading can't be blocked, but almost every bitcoin related services can.