Problem in Internal Order

Hello All

I have a problem closing internal orders, While creating the PO the tax code should have been non taxable or zero tax but they all have Taxable Tax codes now this in turn has caused the purchase orders create a negative po balance which prevents a project from being closed.

1)  How can I close all such internal orders  which is involved in above problem

2) How can I use Only Zero taxcode in the PO for all Internal Order while creating the purchase orders

Thanks

Aryan