Is a person considered sinful if they boycott someone who is committing a sin after repeated attempts to prevent them from doing so, and the boycott has continued for nearly four months?

You must continue to advise your brother with gentleness and kindness, remind him of Allah and the punishment for disobedience, seek the help of some of your righteous brothers to advise him, and gift him some tapes that speak about the danger of sins. And supplicate abundantly for his guidance. As for boycotting, it is the last resort, and it should only be used if the transgressor persists in his disobedience after being advised, taking circumstances into account.