The value of crypto would decrease but never be completely gone. A whole pandora's box was opened and they can't put the genie back in the bottle no matter how hard they try.
If they were to kill the offramps, the ecosystem would move on websites on Tor or I2P that would work P2P, they can use high privacy coins such as Monero or zcash. Or people will trade it in person using escrow systems. It's not like the first BTC trasactions had the privilege of nice exchanges. The possibilities are there. Just the incentives are not yet present.
The software itself they can treat is as malware - blackhole IPs and shutdown any hosts involved.