Is it permissible to continue greeting someone who has committed a major sin, and can the incident of the Prophet (peace and blessings be upon him) shunning the three who lagged behind from the Battle of Tabuk be considered a measure for dealing with disobedient individuals until they repent, and how should one deal with them even if they are relatives?

It is obligatory to condemn whoever openly commits a sin, and boycotting him is permissible if it is beneficial in deterring him, as the Prophet, peace and blessings be upon him, did with the three who stayed behind from the Battle of Tabuk. However, if boycotting leads to a greater harm, such as the boycotted person increasing in evil, then it is not permissible. The Prophet, peace and blessings be upon him, sometimes used boycotting and at other times used conciliation, with the aim of inviting creation to obey Allah. Permissible boycotting includes withholding greetings, visiting the sick, and attending the funeral of one who openly commits a sin. Permissible conciliation includes being kind to them and admonishing them with wisdom and good counsel, as 'Umar ibn al-Khattab, may Allah be pleased with him, did with the man who repeatedly drank alcohol, when he wrote to him reminding him of Allah and calling him to repentance.