No. All interaction with Safeguard is via the API - even the desktop and web clients communicate with safeguard via the API. So disabling the API is the same as disabling the UI.
Which API endpoints the user has access to is dictated by admin role permissions and access policy settings. There are few unauthenticated endpoints such as the swagger URI and the /service/appliance/Version endpoint which provide metadata about the server and allow the clients to detect whether the appliance exists and is available. There is no way to disable the metadata endpoints.