One possibility is to hard fork the chain and convince everybody that the forked version is the real version, including the users and exchanges. (Something like the "fix" for "The DAO" in Etherum.) In the hard fork you can change slightly the algorithm, or make a haircut (like the usual solutions of countries in these situations).

The old unforked version of the coin will continue to drop to 0, and will try algorithmically desperately to sell worthless bonds to try to raise the price, but nobody sane would buy them.

I'm not sure if this is legal or moral, but I would not be surprised if something similar appears in the future.