The best way to complain, is to never give them your custom again. I believe in giving everyone and everything the benefit of the doubt, but if they are consistently letting you down as you say they have been, register a complaint with them, then just don't use them any more. Take your money to places where you consider the food and service to be better. Pizza Hut is such a big company that they just churn out the same rubbish every time knowing that people will be there to consume it. If people stop putting up with what they consider to be negative treatment and go somewhere where they have a much better time, the companies will soon take notice.
Good luck in your quest.