Navigate to NewViews > Account > Expenses on the Database Explorer and select the drop down arrow above the Accounts button tab which will display a selection list. Select Trade Tax Info which will display all of your expense account Tax 1 and Tax 2 statuses. Click on the applicable expense account and press F3 under the Tax 1 Status column. Select Exempt to make the expense account exempt from Tax 1. Do the same for the Tax 2 Status if applicable. From this table you can specify which expense account are exempt from taxes. The default is taxable which is set in the Purchase Invoice Journal.
An alternative to the selecting the drop down arrow above the Accounts tab is to press Ctrl F11 and select the drop down arrow next to current settings in the Define Columns window and select Trade/Tax Info.