One way you could do it, bit of a fudge, would be from the Admin GUI -> Global Settings -> AS/AV -> Subject Prefix set it something like {SPAM}. Then create a filter on peoples accounts that check for this in the subject line and for the action Mark As Read. Would this achieve what you are looking for ? Also, could you update your profile please with the output from
Code:
su - zimbra
zmcontrol -v