You cannot do this on an out of the box SME installation, but with some additional contribs installed.
Install the contribs RPMs smeserver-mailsorting and smeserver-qpsmtpd-spamassassinlevelstars
Set
Custom spam tagging level = 1
Custom spam rejection level = 12
Sort spam into junkmail folder = Disabled
Modify subject of spam messages = Disabled
Then every user must setup two mailfilter rules in the user-manager, one for deleting and another for moving the mail into the junkmail folder depending on the number of asterisks (*) in the X-Spam-Level header field.