Passive-aggressive behaviour, such as ignoring and shunning me. I absolutely hate this. I had to deal with it at high school, where nobody would acknowlege me and would just move away from me, and that cut me up badly. But what is even worse is when I get this from friends and girlfriends. For example: I might say that I was going to meet somebody for lunch but fail to turn up, and even though i quickly ring to explain that I was caught up in traffic, or had an assignment i needed to complete urgently, I get "punished" by having my calls unanswered (or posts unanswered on Skype). This is incredibly childish, manipulative behaviour and is one of the surest ways to make me hate you.
As I've alluded to, I find it especially irritating online as it is next to impossible to chase said person up.