Better wording for account deactivation
When a user want to delete his account, he can think "deactivation" is enough but doesn't necessarily know that it just means that he cannot log anymore…
Maybe (in advanced settings):
"By deactivating your account you will no longer be able to log in but the messages will be kept. To permanently delete your account and your messages, please contact your administrator."
And a similar (or the same) message in email notification.