You can do this on the command line as the Zimbra user:
Code:
zmmailbox -z -m mailbox@domain.com emptyFolder Inbox
Be careful, it will delete
all messages in the folder
An alternative method would be to create a search in the web client, show 100 messages per page, and then Ctrl+A, Del. Incidentally, you can right-click on the Trash folder and choose 'Empty'.