Article 15 of the REALTORS Code of Ethics states that REALTORS "shall not knowingly or recklessly make false or misleading statements about other real estate professionals, their businesses, or their business practices.”
As a REALTOR, Broker, appraiser, past president of our local MLS association and past state board member of the California Assoc. of REALTORS.......I think you'll find it's nearly impossible to get anything enforced.
I know of agents, over the years that would openly say things like "I would never use or allow the use of John Smith appraiser since he always comes in too low" or something similar. They still do. One group of agents in the Fresno area would get together once a month over coffee and blacklist certain appraisers because those appraisers would "not play ball" so to speak.
I realize in some states you have to be an actual REALTOR to be an appraiser. It's not like that in California. Appraisers can obtain MLS data without being a REALTOR. A broker can conduct business without being a REALTOR. Being a REALTOR and falling under the Code of Ethics means you are a member of the National Association of REALTORS. Of course many appraisers are not.
While you can and certainly should contact the local association, the agents broker or office, Et.al, and make your concerns known, I doubt little can and/or will be done to stop agents from bad mouthing appraisers.