Service of vehicle - replace parts without consent

I took my son's Chef Utility for its 30000 km service 7:30 in the morning, with my arrival at 14:50 they were not done yet. at around 16:15 they called me and told me that they have replaced the front wheel drums. When asked who gave permission the acknowledge they did not phone for permission. They then offered that I can pay for the parts only which I did not accept since it was money I did not have as well as the principal of the case. They took the new parts off and placed the old parts back after a huge argument and I only got the vehicle just before 17:00. The wheel drum on the front left have been damaged after it came loose while driving after they themselves replaced the clutch at 7000 km, which was a factory fault. At the time the nut came loose, they indicated that it is impossible for the nut holding the wheel drums to came loose but did not want to take any responsibility for it. During this 30000 km on this brand new bakie we had several problems. now I am asking you is it the vehicle which is not trustworthy or is it the mechanics that ***** stuff up when they service or fix the vehicle?????<br>