Cancel/remove defaulted tax rate from PO

Hi,

I have a default tax rate applied to all Purchase Order transaction but user needs to apply exempt tax for a few numbers of new purchase orders -POs- so,

in other words, how to cancel defaulted tax rate manually from specific POs while creating it.

Note: I have done a zero rate and it's working fine by overriding tax code but, it's not acceptable by user to find a zero tax line.

Please advise.

Thanks and Best Regards;