If someone is sitting in my seat, I always kick them out. Theirs is an act of aggression, and they created the situation, so I will be more than happy to make it an uncomfortable and awkward moment for them.

That being said, for the first time ever in my flying experience someone was honest with me about stealing my seat. On my first flight of 2008 on an EMB 135 I had 9A (exit row) and someone was sitting in my seat. I did the usual double check of my boarding pass and he volunteered "am I sitting in your seat"? I said yes and he said, "I was hoping you wouldn't make the flight on time". Thank you for your honesty. I have never come closer to contradicting myself concerning my opening remark.