1. On your Matter, edit the "Prof. Fees Tax Rate Override" field

NOTE: If you cannot see this field, please contact your System Administrator to have it revealed.

2. Set the amount to "0"

3. Click on Save

4. Depending on your business process, create an Invoice using the Batch Billing or Batch Pre-Billing methods.
5. Go back to the Matter and click on your new Invoice

6. Note that the Invoice Lines have the Tax set to $0.00, which confirms the change has taken affect.