Dear Richard,
You can follow the below link in Knowledgebase to use command to delete 500 messages at a time if you know the first and last MIDs of those quarantine messages. It should still be faster than reloading the GUI to click and select 500 quarantined messages to delete.
http://tinyurl.com/efonf
e.g. If the first quarantined MID is 29137600, add 499 to the number as last MID each time
quarantineconfig deletemsg 29137600-29138099 Policy
quarantineconfig deletemsg 29138099-29138498 Policy
...
...
You may want to write a script to automate this.
Tommy