Canada is the source of more than 80% of the US's potash, the source of fertilizer. Without that, the entire farming industry and its crops withers and dies. It is not something easily sourced and not something easily replaced, and the crops all die long before it can be sourced or replaced. Your suggestion to close the border is absurd, jingoistic, empty, and meaningless.
Again, I'm not an economist, bear with me and explain what that's got to do with the trade deficit being a deficit? You could have a GDP of a quintillion dollars a year but if you're consistently running a trade deficit (which the US has for decades), that strongly implies you cannot close your borders with impunity.
If you could supply the goods cheaper than importing them, people would, no? Simple capitalism!