Is it obligatory to forbid forbidden handshakes every time they occur, or is it sufficient to forbid them once and then leave them, and should the forbidding be collective or individual?

Shaking hands with non-mahram (definition) women is a reprehensible and forbidden act. Enjoining good and forbidding evil is an obligation, and it may become an individual obligation (fard ayn) if only one person knows about it or if only they can remove it. Enjoining good and forbidding evil does not cease to be an obligation merely because one suspects it will be ineffective; rather, it is an excuse before Allah. The general rule is to openly denounce the one who openly commits a reprehensible act. However, secrecy and gentleness may be preferred if it is highly probable that the perpetrator will benefit from such an approach.