We don't have an option for deleting all mail from the quarantine at once yet but you can vote for this feature in our Community Suggestion Box, here:

Delete all messages option for quarantine viewer or Delete multiple dates or pages with single click
(these links may prompt you to login to your SpamHero control panel)

Meanwhile, you can delete up to 500 messages at a time by changing your page size to 500 messages and then using the "Delete Spam" button at the bottom of the quarantine list, as shown here: