How to reset spam_subject_tag using serveradmin?

I use serveradmin to set a tag for the subject line of spam emails, by the command:

sudo serveradmin settings mail:postfix:spam_subject_tag = "***SPAM*** "

In order that I could filter automatically. I no longer need to do this, so wish to reset the spam subject tag. However, I can't find the appropriate value for the key. Neither a double quoted space or even two double quotes works. What I want to do is reset the value of the key to its original value of ""

Can someone let me know the command in order to do this?