Cash is used for illegal purposes. Barter is used for illegal purposes.

Bitcoin is being used for illegal purposes. Ramen is used for illegal purposes.

Ban ramen, Bitcoin, prison yogurt, barter, USD.

Bitcoin is significantly easier to track than cash or Ramen. If you want to get rid of illegal Bitcoin transactions you simply need the government to care. But the real risk is the ramen/USD trade, that can't be tracked. Ban Ramen?