Hi,
You should use the transaction SWIA with the workitem id as selection.
Double-click on the item.
"Goto" >> "Technical work item display"
"Edit" >> "Change"
You'll then see the admin panel like below:
Clicking on "Logically delete" will really delete the Item with approving/rejecting.
Pay attention that you'll need the workflow admin autorizations.
Best regards,
Jonathan