I definitely feel you there
My parents are divorced (and have been for the entire 23 years of my life). My dad never talks bad about my mom, but my mom goes off about my dad. It drives me crazy, because whenever I go visit my mom, she's always going off, and her house is stock full of bad foods and empty calories.
Lately I've been trying to keep myself from getting in between, so I just don't bring anything up, and when that doesn't work, I just simply say, "I love you both, and I don't think it's fair that I'm put in this position. I do not want to speak about this anymore." And if they don't accept it, I just walk away. For you, you could just say, "since you aren't respecting my wishes, I'll have to hang up now. Goodbye." *click*
Easier said than done, I know. But after I do it, I feel a huge weight off my shoulders
Good luck!