Im sorry to say that Yes your servicer is correct the oven temperature control thermostat is NO LONGER available for this old of a model .
The Manufacture only has to keep parts available for 7 years ,then they can delete their stock .
You might be able to find one at a local USEd appliance parts store if you have one in your area .
But That would be no guarantee that it is in 100% working order.
You might also try E-Bay.
But From new Appliance parts dealers it is no longer available.
No replacement or the parts dealers would substitute it already.
And As for the disabling .if the servicer just disables the power into the thermostat ,then it would be NO PROBLEM even it the oven was turned on .