Advice is least heeded when most needed

व्यालं बालमृणालतन्तुभिरसौ रोद्धुं समुज्जृम्भते
छेत्तुं वज्रमणीं शिरीषकुसुमप्रान्तेन सन्नह्यते ।
माधुर्यं मधुबिन्दुना रचयितुं क्षाराम्बुधेरीहते
नेतुं वाञ्छति यः खलान्पथि सतां सूक्तैः सुधास्यन्दिभिः ॥

- नीतिशतकम्

Whoever wishes to reform and guide wicked people into the righteous path by using nectarous sweet words, is actually trying to tie an elephant with the tender stalk of a lotus, to cut a diamond with the delicate petals of a Shirish flower, or to impart sweetness to an ocean with a drop of honey.

You cannot wash charcoal white.
Advice is least heeded when most needed.
Never try to teach a pig to sing.
It wastes your time and it annoys the pig.