git reset --hard HEAD@{1}
There is no way you can delete a pull request yourself -- you and the repo owner (and all users with push access to it) can close it, but it will remain in the log. This is part of the philosophy of not denying/hiding what happened during development.
However, if there are critical reasons for deleting it (this is mainly violation of Github Terms of Service), Github support staff will delete it for you.
Whether or not they are willing to delete your PR for you is something you can easily ask them, just drop them an email at support@github.com
UPDATE: Currently Github requires support requests to be created here: https://support.github.com/contact