These cannot be deleted.
The sent messages folder is simply a reminder of what you sent in the last 5-6 days, after which time the message is automatically removed. Just as with an email, you can't 'unsend' a sent message.
The deleted messages folder allows you a few days to recover anything you've deleted. We do not allow the instant deletion of a message, as this would prevent investigation of any potential problems.
Sent and deleted messages do not count towards your message allowance; (you can have unlimited sent and deleted messages).