Umm Muthanna al-Britannia has documented her defection to the "Islamic State" on social media. From ISIS controlled territory in Syria, she recently tweeted, "I came here to die. I will not leave till I get what I came here for..."
Banning contemptible viewpoints isn't going to stop anyone from having them. In fact, it may provoke a further hardening of those beliefs, and perhaps even a violent reassertion of them by their adherents.
What's imperative is to demonstrate that those who believe it was an act of "true" Islam are wrong.
I spoke to a number of atheists from Muslim backgrounds who are still firmly in the closet.