a. Have them listen to Spicegirls and Hanson for the rest of their lives.
b. We'll have a contest to see who can hold their head under water the longest.
c. Accuse them of world hunger, war, and disease.
d. Put all the rabid dogs in the world in Canada, and close off all borders.
e. We'll test nuclear weapons in Canada.
f. Scratch e., we'll use chemical and bio weapons on them. Nukes would melt the ice and cause worldwide flooding.
g. Dig a trench along the border. Without the United States, Canada would float away into the north pole.