There's no one word or short phrase that will capture all that. If the posture is deliberate, that intent may be expressed by the verb snub or cold-shoulder—but neither expresses a physical posture, only a pointed refusal to engage.
Here are some possible ways of expressing the posture—the first group are neutral with respect to intent, the second express a deliberate snub:
Jane sat next to John with her back him.
Jane sat next to John, leaning toward her neighbor on the other side.
With John to her left, Jane stared right, at the lively argument by the buffet.
John had a discouraging view of Jane's back on his right.
Jane sat next to John, pointedly turning her back to him.
As she sat between them, Jane turned pointedly to George, leaving John baffled and fuming.
Speaking animatedly to George on her right, Jane gave the cold shoulder to John on her left.
Jane squared around to gaze at the buffet, presenting John with her back.
But this is all 'way over in writers.se territory.