I utilize the prefetch-mode for my pop3 accounts. All spam-messages are hold in the user portal but not deleted (only indicate as read) at the pop3 - mailbox.
this is currently the default behavior. You can delete it manually in the End-User portal.
There are two additional features that will be integrated into the next Beta Version.
1) Quarantine Manager Here the admin can view each message, select, view, download, delete and release it.
2) Automatic deletion after n days. This freature will delete old messages automatically from the quarantine after the configured amount of days. Per default 14.