Once an expense has been approved it is not possible to delete it, neither can it be unsubmitted (and subsequently deleted).
One course of action would be to ensure that the incorrect item is not posted to the accounts ledgers - eg by deleting it from the Dimensions download screen - and then ask the user to submit a new expense if required.
It is also possible to enter negative value expenses so, in the case of mileage clams, unwanted mileage records could be cleared.
