Your question is interesting. It is about both excluding and promoting.
SEO and link sharing have been mentioned already. All I can add to this is that having some content and articles that target specific Canadian or US interests or localities may improve your statistics in North America.
A good way to reduce the visibility of your site to large portions of the world's population is to use .htaccess (Apache) or similar restrictions in other environments to block search engines that are specific to non North American markets. For example, on some of my web sites I block the "Baidu Spider" from China, which chewed up a lot of accesses of my sites. This also makes my sites less visible from China, which is OK because I cater to a western market anyway.
Given what you stated, you may wish to block entire ranges of IP addresses that are allocated to particular countries. It's often stated that blocking China by IP can inadvertently block some users in Australia and NZ. You probably have a cleaner challenge pertaining to who to block and you are probably not so concerned about overlapping of countries with IP ranges.
So the answers for you are: partly focused content and SEO, and partly technical in nature - block stuff that increases your exposure to markets you don't want to serve.