Could have, but didn't, because phones are a 1-1 medium and people don't go around cold-calling others to see if they're into white supremacy and conspiracy theories.
Not only that, it wasn't one person, or rather it was not one person alone. He was fed, indoctrinated and enabled by his co-conspirators who egged him on with anti-Islam material. In some sense they got off lightly: if the situation had been reversed and we were talking about an Islamic supremacist board promoting violence against Christians which resulted in a massacre, they might have been candidates for an airstrike. "We kill people based on metadata", remember? https://www.nybooks.com/daily/2014/05/10/we-kill-people-base...
The ban would most likely be for refusal to comply with rules regarding extremist or crime-promoting material. For example the UK has a unit pointed at the major social networks making binding takedown requests against (primarily) Daesh material: https://wiki.openrightsgroup.org/wiki/Counter-Terrorism_Inte...
There are proposals to make the terrorist takedown regime mandatory across Europe: https://www.bbc.co.uk/news/technology-45495544
At the moment the authorities are surprisingly un-worried about white supremacist terrorism compared to Islamist terrorism, of which there is very little on the chans. So they largely escape takedown requirements. But if the extremism escalates that may not stay that way.