How do I delete a message from the Quarantine?

You're not required to delete any messages from Quarantine. They will expire after a month. You only need to take action if you wish to release the message, add the sender to your safe list or add the sender to your block list. The messages in Quarantine are not being reflected against your e-mail account storage quota and are not affecting your e-mail account in any way, so there is no reason to purge messages from the Quarantine.