It's to fool primitive scrapers looking for e-mail addresses with the @ symbol. It's handled like that on the entire KIT website.
* Failed login: I can't tell you if the username/email or password was wrong, since that'd reveal if the the user exists
* Failed signup attempt with existing username/email: Cannot create an account, because a user with that username/email already exists
Don't shit your pants. The IP ban doesn't actually work.