To "blow someone off" means that you ignore their advice, or if you are supposed to meet someone, you don't show up or don't come.
Mary was supposed to join us for dinner, but she's blowing us off because she got a call from that guy she's interested in.
Or That's bad advice - just blow it off. Of course, what a teenager considers to be bad advice is not what a parent considers to be bad advice.
"In your face" or "to your face" means to be blatent about it.
So, in combination, this means to snub them, or what my guilty-pleasure Regency romance novels would call "give them the cut direct."
You drop your 12-year-old son off at school and say "I love you." He completley ignores you - he has blown you off (to your face).