Any service assigned to a staff member and selected to Display Online can be scheduled by a client during online booking. If you wish to display a single service online without allowing a client to schedule, you would need to ensure that the service is not assigned to any Active Staff members, and selected to "Display Online" in your service menu.
You can find more information on Assigning/Un-assigning services here: Assigning Services.
This will allow clients to view the service on your website, but will not give them the option to select a time and date for their appointment if they select it.
Note: MassageBook suggests including information within the service's description to alert your clients that the service is not available for online booking.
If you wish to display all services online without allowing clients to book them via your website, you will want to ensure that all of your services are selected to "Display Online" in your service menu, and that your Online Booking Rules are disabled: Disabling Online Booking Rules.