The government could if they wanted to go after individuals on counterfeiting charges. See also the remark by pjeby about banking laws (although as I said there, that seems hard to pin down). Unfortunately there are a large number of different federal laws about money and taxes, so it shouldn't be that hard to find some federal crime to use. There may also be ways of tying in RICO charges which could be potentially used to then prosecute anyone using Bitcoin based on the most illegal activity of anyone using Bitcoin (so murder for hire, drug running, kidnapping, etc. from anyone on the network could potentially result in charges for anyone on the network.) In general, when the feds want something to be illegal and it involves money, they'll find a way.