Spam does not stay in the Junk folder eprmanently, it's gets deleted after a period of time that you set in the Admin UI. Perhaps you ought to find out why you have spam in the users Inbox, search the forums for some tips as it's been covered before.
You might also want to post details of what you version of Zimbra is, what sort of spam is in the Inbox (post some headers), what your Kill/Tag percentages are etc...
Improving Anti-spam system - Zimbra :: Wiki