When you add item(s) to the current sale a total tax line appears under everything. You can click or swipe (depends on the OS) this tax line and get the option to delete it. I have mine setup to require a code from select people (say managers) so that I can make sure the correct tax exempt paper accompanies the customer's request to not pay taxes.

You can also edit an individual items and uncheck taxes on it as well. This will trigger the same security code process.

Hope this helps!

Moz