Make sure to let them know how much of their products you've bought, and how much their decision will influence your future purchases 
Well duh! I may be a self-important person who cares about self-appointed spokesmen being scammers but I do know how boycotts work.
I'm always reminded of when the Baptists made a grandiose showing of boycotting Disney over gay issues. Few years later, they sent a letter to Disney threatening to continue their boycott, and Disney looked at their income statement, and said, "Uh... who are you again?"