Officially "junk mail" refers to mail that is not addressed to you. To stop this kind of mail, all you have to do is write a letter to Canada Post and they will put a red dot on your mail box. This web site tells you how.
If you have PO Box delivery, you can request your local post office to deliver only mail that is addressed to you.
Eliminate junk mail: save a few trees and keep your box from being clogged if you miss a few days.