In MarketPress, there doesn't appear to be an option to never allow signups to the store. Some of my clients never want customers to be offered a signup.
Please provide a way to disable the signup/login section displayed on the checkout page.
I note that there is currently an option in Products/Store Settings/General Settings/Misc Settings called Force Login.
I suggest there be a similar option "disable logins" which would hide/remove the ability to login and just display the "checkout now" buttons instead.