admonish, warn, prohibit
誡 centers on using words to correct or restrain: admonishing, warning, or prohibiting someone from a harmful course of action.
誡 combines 言 (speech) with 戒 (admonish, warn), forming a character that reinforces the idea of verbal warning or prohibition.
Words (言) of warning (戒) come together: 誡 is a spoken admonishment or prohibition.
For カイ, imagine a stern parent saying 'KAI!' (like a sharp 'cut it out!') to admonish a child: KAI -> カイ, and the warning is delivered with words.