If you lock your romance with Mizz out of the equation, you'll not express any sexual interest in him. This doesn't mean he won't flirt; that's just in his nature! Even an asexual Rael still gets hit on from time to time by Mizz, but he knows that it won't go anywhere.
Ace Rael or an uninterested Rael simply won't ever be given the option to get down and dirty with Mizz, and narrative and dialogue will reflect that choice. None of your choices for Rael will stop that cheetah, though, heh heh. Hope that helps!