cPanel --> File Manager and delete everything in public_html . DO NOT DELETE public_html or www
cPanel --> MySQL Databases and delete any databases/users
cPanel to remove any extra email accounts, FTP accounts.
Use cPanel to remove any Add-on domains. Then any subdomains. Finally, any parked domain you do not want.
That should put you back to your original state.
Do not ask to have your account deleted so you can register for another one.