As someone who is a philosopher and  grew up in East Germany I am horrified by the Kingswell's article. The GDR Government used the same excuse [a small group of actual (and unteachable) Nazis among a rapidly growing number of political opponents] to enact the same kind of thought policing Kingswell uses/proposes. Of course it did not end there, massive surveillance followed, then, in 1961 the wall, officially termed "Antifascist Protection Bulwark', was built. In the name of protecting them from fascism it imprisoned 17 Million people for 28 years! Kingswell is presumably sure this could never happen in Canada. Perhaps. But East Germans were equally sure (as late as spring 1961) it would not happen to them or they would have gotten out while they could. 
History shows: this kind of regulation on free speech is an invitation to abuse by corruptible governments. As philosopher I invite Kingswell to imagine the ideas deemed dangerous were his own - would he still advocate policing them?