A maintenance crew prepared the aircraft to taxi. After engine start up both engines went to flight idle because someone inadvertently pulled the flight idle circuit breaker. The A300 jumped the chocks and headed for the Airport Repairs Cargo hangar. Since the engines were in flight idle braking and nosewheel steering did not work. The maintenance crew tried to reduce power on engine number and failing in that, increased power to 90% on the number 2 engine. The aircraft started to turn and after turning some 80 degrees they lost control of the aircraft and it went straight through the airport perimeter wall next to the hangars. The nose wheel collapsed and the aircraft nose dived into the ground. The aircraft was being taxied by a senior aircraft maintenance engineer with 39 years of experience in the field.