[ponow]

No, Popper there goes beyond that: he requires that people argue rationally and not tell followers to ignore rational arguments. That's too high a bar, IMO. The problem is the followers' use of violence. Initiating "preventative violence" against a mere speaker (who himself did no violent act) is immoral, even if an expedient way of preventing seemingly likely violence by followers. It puts the power of judging legitimate speech in a privileged group, and it is likely to be applied unevenly and primarily on political enemies.