I've run into far too many of these people. This is by far and large my biggest pet peeve in air travel.
Last year I flew DTW/RSW after a redeye from the west coast. I cleared my upgrade and specifically chose the last row of F so that I could recline (and sleep) without going into anyone's personal space. I boarded to see my seat (and the one adjacent) occupied by a husband/wife combo being aggressive in telling me to sit down in front of them in row 5. Since I was too tired to care, I did as requested. Once airborne, I reclined and the husband went bezerk: he paid for his first class space, and I had no right to intrude on it... or so he thought.
I still run into this very often, both up front and in coach. I always just try to board early enough to prevent it, and if there's someone in my seat, I let the crew deal with it.