I don't think there's a way to do that. In fact, I wouldn't be suprised if most users wanted both cores working on rendering in order to get the rendering done faster.
If I were you, I'd invest in a better cooling system (if, in fact, your diagnosis is correct) There are utilities out there to tell you what your CPU temp is.
Take the cover of your PC and point a fan at it- that should work 😉