First, talk to their broker. The owner/manager of the real estate company they work for. That should do the trick and if not, find out what agency licenses real estate agents in Washington and talk to them. If this is related to your prior post, however, your problem may not be so much the realtor as the owner. They may be trying to avoid fines and penalties for doing work without a permit. Talk to the broker, you may be able to work with the local municipal authority to waive fines in exchange for just getting the property sold and avoid urban blight.