First of, what temperature are you talking about? If it's above 10*C (50*F) there are many simple commercial thermostats available - you want the type for baseboard heaters. They could also be tweaked for lower temperature using the setting screw inside them. If you want an inverted function (disconnect below a set temperature) you'll probably need some DIY. Sometimes the internal microswitch has both NO and NC contacts, or you can replace the existing one with a SPDT one. After that the skies are the limits for complexity and overkill.