If a particular service requires a consult, first, do not allow booking a service (ie: laser hair removal) if the client has yet to have a consultation (which our practice requires a consultation prior to getting treated.

Or...create a setting which would allow existing clients to receive certain services and book them on-line while preventing new clients from booking certain services on-line (because they might need a consultation, first OR the service may not be the right thing for that client and they would only know this if they had a consultation).