So what can France and other countries do to stem the tide of such attacks?
Enforce laws on the books and if the penalties are not harsh enough, increase them. It isn't just for crypto crimes, but for any: if you are violent or rob someone, you need to be out of the general population for a long time and learn to control yourself.
In my country, any form of kidnapping case the penalty is death by firing squad, same as arms robbery and raps, so yes you are right let the penalty for crimes be based on capital offenses and not just cryptocurrency inclined in a way, security is a global issues right now and most country are facing the issues with kidnapping and bandatery and terrorisms.