Sorry, but selection limits were purposely removed from admin side manual registrations because many people complained about that. They felt that as an admin they should be able to register any options and override limits seen by users. There is no setting to change this. You might consider having them just do the registrations on the frontend. If there is a payment, have a discount code that makes if free so they do not have to process a payment. Just an idea. Otherwise, if they want to use the backend and this is a concern, they'll just have to be aware of current registration status and what is and is not available. They can run CSV exports, etc to have that info.