I am not sure the degree of terrorism going on in Canada that's ISIS or even vaguely Muslim related. Canada seems pretty open, generally.
Shoot of asking people if they are ISIS or extremist (and I think an infiltrator would lie, anyway) how does one prevent criminals from entering the country when refugees by the nature of their experience often have no or few papers? We let a Catholic doctor in and he molested hundreds of girls. So religion isn't a great litmus test of good character as we all know in this forum.
OTOH, letting known ISIS supporters in would be kind of nuts. Even disenchanted ones still had a mindset that allowed them to believe they could support it. If they are citizens, then I'd think supporting such a group as ISIS would result in prison and a lot of counseling before they could be allowed out.