Then they should create a program where specific individuals can placed on a deradicalization program and be banned from extremist material. And then make it a crime to view the material while you are on that program.
Sure you can - there's plenty of precedent for e.g. sectioning people into mental hospitals for their own safety or removing people's driving licenses when they might be a danger (without committing a crime - bad eyesight, bad motor control, drug regimen, etc.)
Sending people to mental hospitals is a fair example, but that's why I wrote "generally", acknowledging the existence of exceptions. In that case, it's accepted that consent is an oxymoron, which doesn't really apply here.