To make a customer PST exempt, you can assign the Exempt (0%) tax code to their profile, Jeffry.
Here's how:
Go to the Sales menu and navigate to the Customers tab.
Click the customer's name from the list.
Tick the Edit button.
Scroll down to the Additional info section.
Select the designated Checkbox for the Assign default tax code option.
Choose Exempt (0%) as the Default tax code.
After that, hit Save.
Please note that the exemption applies to PST and all available tax codes. But, you can still assign tax for this customer when necessary by selecting the appropriate code under the Sales Tax column while creating a transaction.