turn your back on sb

turn your back (on somebody/something)
a) to refuse to help, support, or be involved with someone or something :
How can you turn your back on your own mother?
In his twenties he turned his back on his Catholic faith.
b) to turn so that your back is pointing towards someone or something, and you are not looking at them :
Angrily, she turned her back on him.

Oxford Advanced Learner's Dictionary

turn your back on sb/sth
1   to move so that you are standing or sitting with your back facing sb/sth
When on stage, try not to turn your back on the audience.
2   to reject sb/sth that you have previously been connected with
She turned her back on them when they needed her.
   Some newspapers have turned their backs on discussion and argument.

Oxford Advanced Learner’s Dictionary, 8th edition