Theoretically, it is possible. There are a few things which the government can do, but none of them are very attractive propositions in my opinion. Let's look at various scenarios one by one. First of all, the government can declare the possession of Bitcoin as a criminal offense and arrest those who are suspected of holding Bitcoins. But this can unleash a legal firestorm for the authorities. Another options is to block various exchanges and online wallets. But if a good VPN is used, these restrictions can be over-ridden. Another option may be to conduct a 51% attack, using the super-computers in the possession of the government.