Using an unsuitable vocal technique can ruin a song, harm your singing voice, might not match the music, and throw another person's performance off. If the person insist on doing this explain to them why it doesn't work. Show them other versions of the song that are different than the technique they want to use and explain to them why these techniques work better. Explain the history of the song and why the song is sung in the other techniques. Try to get them to maybe use that technique in another song by giving choices of where that technique might work better. Ask them why they want to use the technique and give them good reasons why it doesn't work with that specific song. Provide better choices of techniques and show them why it works.

Very often it is better to redirect the attention by giving some repertoire solvable with the suitable technique we want them to find being better and not with the unsustainable option, usually when they get back to the song the fixed badly they are not satisfied anymore by the worst option.