Expense Report Multi-Currency Problem

We have the multi-currency feature turned on but are unable to shut it off (disable it) on Expense Reports.  Essentially we don't want employees to enter Expense Reports in anything other than our base currency.

We've tried finding a global setting (doesn't exist), hiding the column on the expense report transaction forms (can't hide the column-it is mandatory), tried setting a preference under company's global general preferences to disable expense report multi-currency (this option only exists by individual user preference setting and is not feasible to ask everyone in the company plus all new hires to manually set it to F) and we've tried setting the Use Multicurrency checkbox on the expense report to False by using a workflow (doesn't allow this pushbutton field to be set via workflow).