Bad service and follow up

I recently posted about bad service and a bad attitude I received from KFC in Langenhovenpark. Although they did not respond to the post on Hello Peter, they did respond to the complaints I made to the customer care email. They phoned me to ask whether it was handled and what the story was. After I explained the entire situation again, they said "it won't happen again" and hung up. This is again very rude and absolutely pathetic. I do not believe them when they say it wouldn't happen again. I want to know how it would be handled, would the related staff members be held accountable and what can I do when it happens again, because it will.