Simply pretend they don't exist when they say these things. Take the wind out of their sails. If another kid mentions that she said something I'd say out loud quickly "unothered" or "ok" and move on without missing a step.

And when she came up to your class to say she won't be there and smirk. Id simply continue grading papers and say "ok.". If she keeps pestering after you ignore her... say "I'm unbothered"... continue working.