Making an order tax exempt is done on the customer level.
Pull up the customers profile and click the "tax exempt" check box, next click save the save icon.
You will then need to go the event/order and click the save icon. Once you have clicked save within the order you will see the tax is removed.
After you have updated the order you can then go back to your customer, un-check the tax exempt rule and click save again.
**Please be aware that any time an order is updated/re-saved it recalculates the tax based on the customer's tax status. So if an order is updated after the above steps are taken, you will need to go back in and repeat these steps to make the order tax exempt again.