I've posted before about my extended family who,what I have come to understand, are toxic.
I'll try not to dripfeed but will run over the pertinent details. Basically, there was a huge argument at the beginning of 2014 involving me, my DM and another relative. Extended family (an aunt, some cousins and nieces) said some awful things in the run-up, and during. Me and mine decided to cut them off as a result (things were never really smooth before - lots of falseness).
So far, not much after. Then there were a couple of significant family events - one our side; one theirs. I received an abusive letter for ignoring their occasion.
I did not retaliate. I am done.
Today, I was shopping and saw the (ex)aunt. We made eye contact and I looked away. She then made a beeline for me across the space, said that she can ignore me too. I said fine; I have nothing to say. Cue more abuse from her: I'm pathetic and it's all my fault 
I felt myself getting angry, but realise that this wasn't the best course of action. We live in fairly close proximity (I have ignored her and others on around 5 occasions). It seems our silence is getting to them almost a year on.
How should I conduct myself if I bump into anu of them in future? I'm lucky that my dcs were at school when this happened.