Well, paper money is also used for crime. In fact, illegal uses of paper currency are so common, U.S. authorities commonly seize cash from civilians deemed to be carrying too much of it!

In light of this - I think it's rational to remove paper currency from circulation and to move to a digital system where every transaction is tracked to a real person. This could be as simple as using existing banking structures, or maybe some sort of cryptographically-secure, immutable ledger of some sort, tying every transaction back to a real person. Almost like a centralized, managed "block chain".

/s