The first step will be to locate the item on the invoice page. Next, you will need to click on the edit button next to the price.
If the entire event is tax exempt you will need to do this for all items.
Once you do that, there is an "Override Category Default Tax Rates". Select the box to the left of the phrase.
After selecting that box, the tax rate attached to that item should pop up. There is a little "X" next to the tax rate. Click that "X" to remove the tax rate. You will also be able to remove any operational costs if needed.
Once removed, choose the update option on the bottom right to save that setting.
After completing these steps, you should be able to scroll down and see the differences in the taxes area at the bottom of the invoice.