If I were in a meeting designed to fix this issue (and maybe my tech background is the reason I'm thinking this way), I'd focus on eliminating the propaganda coming from ISIS.
How to do this isn't easy, but I think that companies like Twitter, Facebook and other social media services need to step up and admit that their services are being exploited.
As an example, Twitter should disallow access to their services from any ISIS strongholds. They need to do a better job of censoring their services. There doesn't have to be "freedom of speech" on twitter.
From the initial reports, this guy was just a total loser. He wasn't a devout muslim, he wasn't overly religious. He was angry and a common criminal. So then instead of committing suicide like he probably wanted to anyway, he reads some ISIS propaganda and decides that he can kill a bunch of infidels and get to heaven, or if he didn't believe in that, kill a bunch of people that lived in a world he hated.
Bottom line, you have to cut off people's access to ISIS propaganda. That access happens on the internet. In a worst case scenario, you shut the entire Middle east off of the grid, only allowing access on a case by case basis, or something of the sort. This is all quite possible from a technological standpoint.