Have you ever been to something like an office party, family holiday gathering or high school reunion and there was that one person that you really wished weren't there? The worst thing you can think of is actually having to engage this person.
You can't really get away from them because you're in the same space, so you do little things to avoid contact. You keep your hands busy or get that really serious look on your face like you are contemplating something important so that when you actually end up coming into close contact, you can seem to be preoccupied and therefore not capable of acknowledging their presence.
My whole day was like that: