I was asked to stop a change requestor from approving their own changes. as they still need to approve other changes it was not just a case of removing them from an approval group.
I did look around and I did not see much that would help, so I ended up with the process in the attached document
In essence
A user can request a change.
If the requestor is added as a manual approver, the approval request is removed and a message is displayed on screen
If the requestor is added as an automated approver via the workflow, the approval request is changed to "Not Valid for this change"