A small town newspaper in Ontario offers this tip for Canada Post customers:
Canada Post will not put (junk) mail in your box if you request a red dot. This dot is placed at the interior of the post office for the sorters to see. I have had said dot for years and have greatly reduced my household waste.
On its web site, Canada Post say the service is available at residential boxes as well:
To stop getting unaddressed advertising, simply put a note on your mailbox stating that you do not wish to receive Canada Post Neighbourhood Mail™(formerly Unaddressed Admail). Place the note in or on your mailbox, or on the inside lip of your community mailbox, group mailbox or postal box.
The service applies only to mail not personally addressed to the customer.