Yes, in some cases Greenback can help to "back out" or undo previously exported transactions. Please contact Greenback support for assistance.
Note: However, there are some cases where Greenback will not be able to automate the removal of previously exported transactions. For example, Xero does not allow 3rd parties to remove reconciled transactions. Therefore, Greenback cannot un-export the transactions that have been marked as reconciled. You could choose to un-reconcile transactions and then engage Greenback to remove them from Xero.