I think that the problem come from the checkout workflow, to don't have the registration form anymore during the checkout you'll have to :
- go to "Hikashop -> system -> configuration -> checkout"
- set the "Advanced checkout workflow edition" to YES
- and remove the "login" from all checkout step.
The "registration" option will NOT remove the whole registration form. It will remove the username/password fields from the form though and won't create any user account when the user enters his email address and address. If you want to customize the address fields, you can go in the menu Display->Custom fields.
If you want to completely remove the form, you can do like mohamed said, however, it means that you won't get ANY information from your users. No address, no email address, nothing.