Business 102, something that's obvious to anyone who's ever worked a customer facing job but incredibly elusive to a lot of people who went straight to management, is that some customers are net negatives and you should not cater to them. They have some combination of ignorance, stupidity, and/or malice which results in them costing more employee time and/or "goodwill" than you'll ever make from selling things. Your business is better off not having them, and even better if they go off to inflict themselves on your competition.