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.
Which is why people should always learn to use their own wallets and stop relying on web wallets no matter how convenient they might be. When online services are blocked, you still have some options to use and trade BTC if you have access to your own private keys. They can even try to block port 8333 but you can still use a client through TOR.