You can set a project to be tax exempt by first creating a tax discount of 100% in company settings. To do this go to Settings > Company > Currency/Tax. Scroll down to the bottom to Tax Discount. Click ' +Add Tax Discount' :

Name the tax discount 'Tax Exempt' (or whatever works best for your workflow), select to reduce tax amount or tax rate by 100%. You can set this tax discount as default by checking the 'Use By Default' box so that any future projects will default to tax exempt. Click 'Add'. 

To set any currently existing projects to tax exempt open the project and go to Project Details. Scroll down to the Tax Discount section and click the dropdown menu. Select 'Tax Exempt' (or whatever you have named the discount), then click 'Update Project'

Note: Follow these same steps to create and set any type of tax discount.

Let Support know if you have any questions!