What is the ruling of Islamic law on not returning a greeting and shunning a person, despite understanding that the cause of the dispute is trivial and despite repeated attempts to end the quarrel?

Allah the Almighty and His Messenger have forbidden desertion and abandonment. An exception to this is abandonment intended for discipline, for a legitimate religious benefit. Returning a greeting is obligatory by consensus, while initiating a greeting is a Sunnah according to the majority. Deliberately refraining from initiating a greeting, intending abandonment, is forbidden if it exceeds three days. Enduring the harshness of others, striving for communication, and responding to evil with good carries immense reward. It is advised to counsel this person and explain the seriousness of abandonment, and to seek means for reconciliation between you, along with supplication.