Overbooked flights

I am booked on a flight leaving CPT to JHB scheduled for 4:15. I arrived at the airport to find that the flight has been overbooked and I have been bumped off the flight. This is totally unacceptable. The next flight offered was at 19:30pm a whole 3 hours later. We requested an earlier flight and had to refuse to leave until we got an earlier confirm flight. The service is really pathetic.