There is a Cydia tweak called DeleteMail that adds a "Delete All" button to the mail app. I just tested it on my spam and trashcan folders and it worked great. It's 99¢. I'm still on 4.3, though. It may not be iOS 5 compatible yet.
You know you can tap edit and tick the boxes next to each email and select them all for deletion, right? It's not quite as convenient as the jailbreak tweak I just posted, but it's much faster than deleting each email individually and it's a stock feature.
So I know this thread is from three years ago but instead of starting a new thread, I thought I would add to the discussion. If we jump ahead to iOS 8 now, there is still no way to "Mark all" messages and move them to the trash unless there is a way that I have missed. You can mark all messages as read or flag all messages but there is no delete.