@Bonnie335 Since the booking was placed well after 14 March, the guests won't exempt from the terms of your cancellation policy even in the event that the border remains closed in September. (That's the policy, anyway - we all know how inconsistent Airbnb has been with applying it).
The guests took a gamble by booking an international trip under such volatile circumstances, especially if your cancellation policy is Strict. But where we'll be with the pandemic over a month from now is anyone's guess - every booking is a gamble at this point, including domestic ones. Leaving Instant Book on was also a gamble. If you're only willing to accommodate Canadian guests, even if the border re-opens, you'll have to disable IB. If you say anything that expresses a bias based on the guests' nationality, you risk running afoul of the discrimination policy.