Hi Peter - you make some good points.
Perhaps something like you have in public relations - perhaps you join an institute (or a new branch of Moz) by paying a fee and signing up to a code of conduct - if a client is unhappy with your conduct, they can lodge a complaint and challenge your position as a member of the said organisation. That would be a great way forward and restore some level of trust in the industry. A kind of self-regulation if you like.
