What should a Muslim do towards someone who physically assaults him or severely insults his mother, and does boycotting him contradict the Hadith: "Whoever quarrels with his brother for more than three nights, his deeds will not be accepted from him"?

It is not permissible for a Muslim to abandon his Muslim brother for more than three nights, unless there is a legitimate benefit from such abandonment, such as deterring wrongdoers and innovators. If abandonment leads to increased aversion and obstinacy, then the most effective means to reform the transgressor must be used, and this varies according to individuals and circumstances. Forgiveness and pardon are recommended, as Allah Almighty says: "But indeed, if anyone shows patience and forgives, that would truly be an affair requiring strong resolve" and "So overlook with gracious forgiveness." If you decide to abandon someone for a benefit, let it be proportionate to that benefit and not for satisfying anger.