Yes, your username is too long in that case, and it's all a matter about how to read and respond to threads rather than something not being of help. Learning how to reply on forums is something that comes with time though so that's fine. You can also modify posts instead of double posting by clicking the modify button on your post.
We cannot directly delete accounts this way, so if you want to use the same email address again for another account you must first change which email that is linked to it, otherwise the registration will not accept any new potential account. You can change the address linked to an account by logging into the yellow box next to the banner and clicking "User details", there's a field in the next page that loads holding which address that is currently linked to that account.
