I would just say I cannot eat it for health reasons, as I am pre-diabetic. I firmly believe that if I didn't go LC that it would be WHEN, not IF, I get diabetes.
With that mindset, a cheat becomes an ultra-serious health concern, and makes me less likely to stray when offered those "treats".

I have absolutely NO INTENTION of cheating (after all I'm only cheating myself) during the Halloween-New Years craziness.

I view it as being a vegetarian... it's a lifestyle they chose for whatever reason.. they don't go around saying.. "I'm going to cheat with some red meat today!!"