How to change a defaulted "Determine Tax Rate" in a tax rule

Hi, We try to change a "Determine Tax Rate"  in Tax Rules but the field is disabled. We want to create AR invoice (transaction) with multiple lines in which some lines tax clasification code should be null. Even we create a line without a tax, it is defaulting by the rule, therefore have to select %0 tax.

How can we achive this, any suggestion?

Thanks.
Evrim.