How to delete account?

My teen daughter set up a Yahoo account last year without my knowledge. She registered on some comprising websites and is now harassed at school because when her name is searched on Google, the sites appear she is a member of. She has no recollection of the account name or password and the email address she used to open Yahoo has since been canceled. I've tried everything to access the account to cancel it, with no luck. Please help! She is afraid family will see these websites.