I've had a foot on my armrest from the person behind me several times. The first time I pushed it away a few times, but it came back.

The next time I was more aggressive, pushing it and then putting my computer on it and pushing down until it left. It didn't come back.

However, this weekend I witnessed the best technique. geoffco noticed a bare foot on his armrest. It seriously looked diseased, perhaps with athelete's foot or some other ailment. He was drinking water at the time. He instantly starts liberally pouring/sprinkling water on it until it went away. As soon as it is gone from the armrest, he quickly opens one of those alcohol hand wipe things and wipes down his armrest. The rubbing alcohol smell filled the air. Both of us were in the middle of simultaneously shaking our heads in disbelief, along with laughing silently to ourselves with what he had just done.