you can disable the registeration if you do not want to have any account.

1. easy way to disable account is using an extension : [link to disable registration extension](http://www.magentocommerce.com/magento-connect/medma-disable-customer-registration.html)

2. create module if you do not want paid extension : [How to disable frontend registration in Magento](http://stackoverflow.com/questions/2959131/how-to-disable-frontend-registration-in-magento#2961418)

3. create module is too complicated ? Go to path: 
> app/design/frontend/{package}/{theme}/template/customer/form/ 

 find the login.phtml and register.phtml, create two files if not exists and change the content