I have seen countless posts claiming that the government can/will ban bitcoin. What I have never seen is a post stating how it could be done. Until someone can demonstrate how it could be done I will continue to ignore those sophomoric musings.
Theoretically it can't be done except through some lesser known, or impractical, or very expensive attack vectors:
a 51% attack(costly), a FRONT Attack(next to impossible, and costly), or shutting down the entire internet(impractical). These are the unlikely the scenarios. Likely scenarios would be banning bitcoin mining, and seizing mining equipment.