Search for "thermostat", or contact thermostat or surface thermostat, something like this one. Its the old principle of bimetalic component which swings one side or the other because each metal has different thermal dilatation coeficient. It has a temperature from which it closes and one at which it opens the circuit. Depending on their order these can be normally open (NO) or normally closed (NC) - you probably want the NC, and it will open (disconnect) at its opening temperature, which should be the max temperature +some hefty margin (coz its a safety measure, not to be triggered everyday).
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