What should be done to deal with someone who prays and curses in order to bring them back to the right path?

Slandering a Muslim without a just cause is a transgression (fusuq). Slandering the religion or what the religion venerates is an act of disbelief (kufr) that expels one from the faith. The slanderer must be advised and reminded of Allah, and encouraged to repent. If he does not heed the advice and persists in slandering, then boycotting him may become necessary if it serves as a deterrent for him.