Remove the top cover of the oven and identify the programmer 28-pin integrated circuit chip which is mounted on the front panel PCB. This seems often to be called the timer.
Cut the supply connections to the chip, ie. Line connection - brown wire - going to pin L, and the Neutral connection - black wire - going to pin N. Safely tape the cut ends with good insulating tape or heat-shrink sleeving, but caution, the wires may only have PVC insulation so excessive heat from the hot-air blower may soften/melt it.
Ken E
August 2020