WARNING This is to totally start over for your account. Think about each step, as there may be something in that account you don’t intend to lose. WARNING
If you can get to the panel, go to:
MySQL Databases. Delete the databases, and hosts (related to your WordPress installation and domain. You can retain the host with no ill effects)
Manage Domains. Delete the domain (that your WordPress was installed in)
Manage Users. Delete the user (Only if this user only had WordPress installed in their home directory)
Manage Email. Delete the mailbox (Only if you want to wipe out your email too, but this is unrelated to WordPress)
When reinstalling any of these, you might have to wait until the old one is completely cleared from the system.