The WHO actually recommends against closing borders to infected countries. This is because Canada would lose track of who makes it into the country from there. For example, if I were from Italy and needed to get to Canada, and the borders were closed, I would just book a flight to France or some other non-blocked country and then fly to Canada from there.
This is one of the major reasons why Italy is now infected. They lost track of the virus after blocking its borders to some Asian countries.
If everyone closed borders to Italy though, you wouldn't be able to do that.
They could also up their screening process. For example, if France still has open borders with Italy, check where passengers have been prior, maybe screen people who have Italian passports, etc. You wouldn't be able to get everyone, but it would still reduce the number of people getting in.
As it is, I don't believe Canada really has track of anybody. Even if they did, so what? 100 guys show up with the disease, but they are still asymptomatic. By the time they show symptoms (assuming they all do) they've already possibly infected tons more. It's a game of catch-up you can't win.