The article says that the person behind this campaign of harassment has been arrested, tried, and sentenced to five years in prison.

I think what worries people is the low effort it required. This guy was unlikely to strap a bomb to himself or hijack a plane, but he could do considerable damage from behind a computer. The asymmetry of personal sacrifice and external harm is such that fewer extremists are deterred from participating. The solution is to remove the asymmetry. In most cases, the police could solve this by simply slowing down and thinking for two seconds before shooting someone.