What you are looking for is a bi-metalic snap
. The best know devices are called "Klixon" switches. They are trademarked name by Texas Instruments. See: http://www.sensata.com/
for more than you will ever want to know about Klixon switches.
You will need to select a switch
that is normally closed and opens on temperature fall. I'll suggest a switch
with a pair of mounting tabs to attach it to the flue pipe with sheet metal screws. Be sure to select a switch
with leads so you don't have any exposed contacts. You will need to know the desired switch
temperature and differential. Klixon 20420 & 20425 Series in surface mount version would be an appropriate choice.