More likely, they could run a bunch of PR campaigns and plant news stories about how risky and unstable Bitcoins are -- either in an attempt to scare consumers

More likely, they could run a bunch of PR campaigns and plant news stories about how risky and unstable Bitcoins are -- either in an attempt to scare consumers away from the currency, or to actively destabilize the speculative market for the currency. "Banning" the currency seems beyond the reach of a government unless it's prepared to place restrictions on internet usage in general (a la the PRC). This is because, even if the U.S. government (for example) were to declare that dollars are no longer convertible with Bitcoins, people could still trade dollars for BCs, and vice versa, under the table.