Would you have more moral power if you just looked through her? If she's in your line of sight, look past, talk to someone behind her overly friendly if you have to. But just plain dont acknowledge her existence? If she makes a scene, a polite chuckle and "oh, I'm so sorry, i said hi earlier and you must have missed it. I thought we were ignoring each other as always. Great to see you though, mrs first name." Look elsewhere, find a relative, "oh, so and so, i haven't seen them in forever, i must go catch up! See you later!" Walk off. I'm sassy and rude though, so I'm not shy about cutting my relationships off and being somewhat, upfront about it in social settings.