1. Click on Incoming - Spam quarantine. Spam quarantine page will be displayed a list of existing quarantined message will be shown.
2. Click on "Remove" from the dropbox of the left of the message to remove it. If you wish to remove multiple messages at once, tick the box alongside each message  that you wish to remove and select, Remove from the "--select action--" dropbox at the bottom of the page - and click Apply. Those selected message(s) will be removed from the system completely.