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.
Uh, yes it can definitely be made illegal--the same way child pornography and drugs and all that other stuff that bitcoin can buy is already illegal. Mostly globally, too. That doesn't mean people will stop using bitcoin of course, just like they won't stop using the two things I mentioned either. It will just be illegal. Just like it's illegal to print your own money.