hello! your post is a bit old -- hope it's not too late to help. we were struggling with a similar issue and stumbled on your question.
start from "Availability" then in the center of the group (near the calendar) click "Edit Availability/Limits", then (second from the top) "Client Scheduling Limits", then (lower right) "Accept a maximum number of appointments" then "Max number per day of..." and enter 1!
al