In the postage section you can exclude countries or regions such as the whole of Africa.

 

And under account - site prefs - buyer requirements block buyers that have a delivery address to excluded countries.

Blocking messages from blocked buyers is a separate action, but it doesn't truly block messages, it just removes the contact seller link in listings (and doesn't even always do that, I've found).

If they go to a member's profile, they can still contact from there, though when it works, the block will tend to cut out most contact.

Make sure to block any buyers from Canada

 

Their Customs processing is notoriously slow - 4 weeks is about par for the course -  your buyers will open INR cases, and get a refund whilst the package is still within Customs.