Interesting question. I agree to say, no, they can't. Unless they've created it. (and that nobody seems to know for sure) What governments, or banks for that matter, can do is to shut down bitcoin to fiat(cash) exchanges. That might scare a lot people, but in essence it would change nothing. The only way to shut down bitcoin is to shut down the entire internet, which would shutdown the (financial and almost everything else in the) world too.