The best way to put a damper on terrorist acts like this is called counter-terrorism. It is not the same as anti-terrorism, but hunting down terrorists and killing them in ways that other terrorists and their supporters find horrifying.
Oddly enough, they are killed using ordinary means, but then their bodies are used for effect, instead of treated respectfully. As such, there are healthy amounts of psychology in use.
For example, this killer “got away with it” by being sent to prison. But what if he was kidnapped from prison, killed, and then his body was displayed wrapped in a pig skin, with a supposed ‘ritual’ performed to assure that he went to Hell for all eternity?
And his family and friends, that celebrated his terrorism, start getting very choppy and difficult to hear “phone messages from Hell”, screaming blasphemous things from his tortured soul. Like “All Muslims are damned to Hell!” and “Killing the innocent is never forgiven!”
If done properly, it is untraceable, but is intended to be unnerving to such barbarians.
That son of a pig Merah was killed by French cops not long after his murders.
“The best way to put a damper on terrorist acts like this is called counter-terrorism.”
Sadly, your solution is bang on. In view of government’s failure, I’m more than a little surprised that citizens in the UK and Europe haven’t taken a DIY approach.