Refused entry onto flight

We were checked in onto our flight on 1 dec 2017. The Gautrain was delayed by a few mins. We called ahead to say we might be a few mins late. We were told by staff that we would have no problems. On reaching the airport we proceeded straight to the gates which still said "Boarding." We gave in our boarding passes and were about to walk through when another attend came and said that we were too late and that boarding had closed 5mins ago. We were not allowed on the bus and the lady closed the gates on our faces. It was a terrible experience and completely *********. I am sure that she privately sold our tickets to somebody else.