a futile effort; something that has no effect (literally: a clamp on tofu—ineffective on something soft).
彼を叱っても豆腐に鎹だ。聞く耳を持たない。
Scolding him is like putting a clamp on tofu—he won't listen.
あのチームに変更を提案しても豆腐に鎹になるだろう。
If you propose changes to that team, it will probably be a waste of effort.
説得のために何度も会ったが、結局豆腐に鎹だった。
I met many times to persuade them, but in the end it was all for nothing.