The output batches from an ad-hoc WO are almost always being used to satisfy a demand up the chain, so it is not usually possible to delete the WO itself.
The procedure needed is to go into the detail of the WO, then go to the Schedule tab, clear the Ad-Hoc Order Until date and save the WO. It is then removed at the time of the next full reschedule – either automatically by the Agent overnight or if you decide to do one manually.