Do a “iisreset” from the command prompt and this should clear the job. If you have WDM 4.5.1 you can right click on the scheduled job and move ti to an error state. Once the job shows up as “error” with a red icon it can be deleted.
If you have the issue where there is no scheduled job but you can not push a package out to a device as WDM still thinks there is a scheduled job for the device you will have to edit the SQL table. This is unusual for this to happen but I have had to install SQL Enterprise Client Tools so I could edit a table to do this – I can’t recall the specific table but can check it out if you need to,