Taxes are an important part of your business and I'll be happy to share more information on this.
Once you've activated the automated sales tax, you can no longer turn it off. You can edit the customer's tax information if you don't want taxes calculated on the invoice. Let me guide you with the steps.
Go to the Sales tab and click Customers.
Select the customer
Click the Edit tab.
Open the Tax info folder and tick the This customer is tax exempt.
Give this a try and if you have questions, I recommend contacting our support team using this link.