Turning off Tax Calculation for iExpenses

Hi,

We have a client in the US, where they need to have only self assessment of tax for I expenses, but automatic self assessment should be turned off. At the moment based on the config we have, when an expense report is exported to AP, it automatically calculated self assessed tax although there is no tax codes selected when submitting the expense claim. Therefore what is the best way to turn off automatic tax calculation for expense reports after exporting to AP ?

Thanks and Regards,

Migara