You could exclude by proxy by charging a very high shipping rate that nobody would pay. That is the only way to do it. If someone by change orders a $10 item and pays $200 shipping, you can just cancel it.
With that said, I would strongly consider using calculated UPS rates if they are available to you. You will still get sales to urban areas where the shipping cost is similar to Canada Post, but it will accomplish the same thing I outlined in the first paragraph. It will exclude rural areas by proxy, because the calculated rate to those areas will be too high.
The only hitch with calculated shipping is that you have to know the weights and dimensions of your items when they are packaged. You could consider separating your items into two or three pools based on what you'd expect the ceiling to be for them to weigh and then just overestimate the weights at what the ceiling might be.
An example of what I mean. Let's say you sell DVDs. They typically range from 100g-500g and 24x19x2 CM to 24x19x5 CM in a bubble mailer. You could just do 499g 24x19x5 for everything instead of weighing each individual item. Yes, you risk (slightly) overcharging on the calculated shipping, but if the other option is to not offer shipping at all because you don't want to weigh 200 items, the tradeoff is worth it. You can also always issue partial shipping refunds to customers if needed.
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