silent treatment

–noun an act or instance of maintaining silence or aloofness toward another person, esp. as a means of indicating disapproval or rejection.

Another definition:

Maintenance of aloof silence toward another as an expression of one's anger or disapproval: "The rest of the class gave the tattletale the silent treatment."

if a group of people send someone to Coventry, they refuse to speak to them, usually in order to punish them "The other workers sent him to Coventry for not supporting the strike."