Pretty simple. Just file a complaint with your town. The owner will receive a letter that they are in violation of XYZ (almost every town has regulations that say stuff like "grass must be kept shorter than 10", etc), and that they have X days (usually 7, 10 or 14) to remediate. If they don't they will be fined. If they continue to not take care of it, the town can fine them more heavily in court, etc.