Is one sinful if he does not greet a person who shuns him and does not return the greeting?

Allah Almighty has forbidden estrangement and abandonment among Muslims. Therefore, your colleagues boycotting you without a legitimate reason is unlawful (haram), and their not returning your greetings is impermissible, and their committing that is a sin. As for the ruling on reciprocating this by boycotting them and refraining from greeting them, it is permissible for you for only three days, and not permissible thereafter. The better of them is the one who initiates the greeting. Repaying evil with good increases affection and protects against the evil of Satan. There is good in forgiveness, and it increases the servant in honor and dignity, and it is a way to attain Allah's pardon and forgiveness.