You can configure your Community Portal to prevent guests from self-registering. This will prevent external users from registering as full users in your Community Portal and disable contact creation during a guest checkout.
To disable external user registration for your Community Portal:
Navigate to Setup > Communities > All Communities.
Select Workspaces for the active Community Portal you wish to disable external user registration in. This will open My Workspaces.
Select Administration from My Workspaces. This will open Settings.
From Settings, select Login & Registration.
Scroll down to the Registration Page Configuration section and uncheck Allow external users to self-register.
External users will now be unable to register as full users.