They could try to forbid Bitcoin outright, but if they did that it would go underground and stop nothing, which would expose their own impotence.
I do not think they can forbid Bitcoin, because all the negative things (money laundry, drugs, illegal payments, criminal activities) also apply to CASH. They haven't forbidden cash money right?
I'm pretty sure they would like to, though.
Maybe that's why they mentioned this fact about cash in the paper.