Stick a piece of used chewing gum on the foot.

Seriously...if the wandering foot somehow comes in contact with something cold, damp and wet, it usually disappears, at least for a bit. Keep obsessively 'cleaning' the arm rest with a really sloppy wet, cold wipe and don't worry a bit if you accidentally slobber your wet rag on a foot that happens to be on the arm rest. Especially if you manage to dribble a little liquid between his/her toes...