What do you think can be done to mitigate the risk of cryptocurrencies being used for criminal purposes? What measures should be taken? 
- Modify the source code so Bitcoin transactions can only be sent with confirmation of your retina
- Enforce new address generation based on DNA samples
- Only confirm a transaction if the receiver completes a 15-pages long form about the purpose of the transaction
We don't really need anything. I'm quite sure most of the largest hackers have been caught due to not knowing WTF they were doing. Bitcoin ledger's transparency makes it easy for hackers to reveal themselves lol. Our lovely governments have taken the worst measures already. KYC, AML, seizing the wrong money, imposing strict surveillance and control everywhere.. it's happening, and they always say it's the best thing ever against terrorism and other criminal purposes. But guess what? It never works as a tool against crime but as a tool against your privacy.
Stop taking this thing for real, the money you own inside that fiat wallet is so much dirtier than my Bitcoins.