There was a woman at work who did this to me, years ago. We had been good friends, hanging out in the same clique for about a year, and when she decided to punish me for a hilariously (in retrospect) arbitrary reason. As part of this, she pretty much cut me off except for a biweekly guilt trip and constant manipulation of mutual friends.

Whatever. THEN, she got a job in my department. Took a demotion to do so! Then, since we had flexible hours and I always worked 6 to 3, she started working 6-3 too! She loved sleeping in and always worked the 10-7 before, so I knew she was making a big sacrifice to be in personal space, not talking, just.... glaring. THEN, she got her seat changed so she was sitting right behind me.

At first I was amused, but after a while it felt really oppressive. So, since we were the only people in the department for two hours, I started greeting her every morning and asking if she wanted coffee whenever I went downstairs to get her some. It was awesome. She'd just glare, and not answer me. Every. Single. Day. I'd play music, too. Softly, but she could hear it, since she was right behind me. "Do you mind? I can turn it off if you want to." No answer, just a glare. "OK, if you don't mind I'm just going to turn it up."

Another person here mentioned that being happy is "kryptonite" to narcissists, and I'd add that making sure that happiness is out of their control is a big part of it. I made sure our mutual friends knew I was happy (she always asked about me, despite refusing to talk to me even though we spent 40+ hours a week three feet apart), and always tried to act cheerful during her two-hour morning glare-a-thon. Eventually, she started working 10-7 again and changed seats, then everyone got laid off because of 9/11 the end. When Facebook happened, I found out that she was STILL talking about this random, arbitrary thing I'd done to piss her off. Which came down to me and another friend dating for a few months, happiness that was outside her control.