The recommended process for deactivating uses from the common admin doc (chapter 11) is as follows:
Ending User Delegations
To end a user delegation:
Choose User Manager > Delegations. The Delegations page appears.
Click End Delegation. If the delegation had already started, it is stopped immediately.
Use the following procedure to terminate a user that has permanently left your company.
To terminate a user:
Lock the user out of the system. Locking a user prevents the user from logging in to the Ariba
Delegate the user’s authority to another user.
The delegatee can act on all in-process approvable documents in the same way as the locked user
(approve, receive, reconcile, and so on), but cannot create new approvable documents.
For information on closing long-running approvable documents, see the user guide.
After all in-process approvable documents have been processed, end the delegation.
Deactivate the user. When you deactivate a user, the user is no longer active or visible in the Ariba
Monitoring User Activities
To monitor user activities:
Choose Site Manager > Audit Log.
Click List All to display all user activity, or filter user activity by event type or date and click Search or
press the Enter key.
Assuming you` did not follow that process you can find out who has approvables out in the system. From the Analytics screen you can select the Pending Approval fact table and add the approver and requestor (preparer?) to the report and look for the terminated employees. You can do the same from the Approval History fact to see what might be out there already approved.
Hope that helps.