Akitas

I have a 14 month old Akita named Tacoma. As a puppy, we socialized him all the time. He went to the dog park and meet people a lot. Then one day, he just snapped at some woman who met him many times before. Ever since then, there are many people that he will not allow to approach him. He will not growl every time, but he will snap. We have tried the "do not look at him," but many people do not follow those directions, or their attitude completely changes around him. He is truly a handsome dog, and people just want to pet him so bad. He is in advanced obedience, and the trainer cannot see what I am talking about because Tacoma is a different dog at class. I love him so much, but it bothers me that I cannot have people come around him for fear of what his reaction will be. Any suggestions on what I could do to show him that people are not a threat and that they are alright to have around and to pet him?