Thought so, had to boot in to DOS.
To save you the trouble of trying, although 'asset /s /d' is mentioned on a few sites to delete a service tag it is not one of the options listed using /? and it does not work.
I guess whoever wrote that it works thought that because 'asset /d' deletes the asset tag and 'asset /o /d' deletes the owner tag that it would be the same for the service tag.
Incidentally whatever you put in the owner tag appears above the Dell logo on the boot up splash screen - it also appears you can change this as much as you like - I have done it a few times now.