If you disable "guest checkout" in magento -- and are using PayPal -- guest check is still possible via Paypal button--it will show below the "Proceed to Checkout" button.
In order to completely remove the possibility of Guest checkout, you'll need to alter the following:
- copy link.phtml to link-01.phtml or whatever
- open/edit link.phtml and delete (or comment out) everything.
Note: I use numbering for different version backups. Higher number = previous version; e.g. link-01.phtml, link-02.phtml. The reason I don't name it something as such, "link.phtml.bak" is because your code editor will not be able to distinguish what type of file this is if you change the extension (in this case php)... so you won't get the benefit of code highlighting.