In my opinion, leaning away from someone when they start invading your space is just condoning and enabling their bad behavior. I used to lean away too, but finally one day I just became so fed up with elbow-pokers and leg-splayers that I made a decision to ignore them and just sit however I want, even if it means I am splaying my legs back at them or bumping their arms with my elbows. Sometimes, when I "accidentally" bump them back, they get the message and withdraw back into their space (at which point I will then make efforts to stay in my space and not infringe on their comfort). When they do not, then I take it as an invitation to be equally thoughtless and I just sit however I want, in whatever way makes me comfortable, regardless of whether I am poking or bumping the other person.