First add a column to your cart: Category
You will have to update the add to cart on the results and detiails page to make sure that the category is stored from the database in the cart.
Create a Calculation in your cart object:
TaxableValue = (([Category]!=1)?[TotalPrice]:0)
Then you can create your tax rule based on the total of the TaxableValue column instead of the TotalPrice so that it won't include items with Category of 1