You cannot wash charcoal white

स्वभावो नोपदेशेन
शक्यते कर्तुमन्यथा ।
सुतप्तमपि पानीयम्
पुनर्गच्छति शीतताम् ॥

- पञ्चतन्त्रम्

It is impossible to change the inherent nature of people by advice. Even though water is heated to boiling, it gradually turns cold by its nature.

You cannot wash charcoal white.
Advice is least heeded when most needed.