You can't polish a turd

न दुर्जनः साधुदशामुपैति
बहुप्रकारैरपि शिक्ष्यमाणः ।
आमूलसिक्तः पयसा घृतेन
न निम्बवृक्षो मधुरत्वमेति ॥

- चाणक्यनीतिः

A wicked person never turns virtuous though advised in several ways. A neem tree never attains sweetness even if wetted with milk and ghee thoroughly from the root to leaves.

You can't polish a turd.
Never try to teach a pig to sing.
It wastes your time and it annoys the pig.
You cannot wash charcoal white.