Answer a fool according to his folly

लभेत सिकतासु तैलमपि यत्नतः पीडयन्
पिबेच्च मृगतृष्णिकासु सलिलं पिपासार्दितः ।
कदाचिदपि पर्यटन् शशविषाणमासादयेत्
न तु प्रतिनिविष्टमूर्खजनचित्तमाराधयेत् ॥

- नीतिशतकम्

It may be possible to extract oil even by squeezing sand. It may also be possible to quench thirst by drinking water from a mirage. One may even find horns of a rabbit possibly while traveling. But it is impossible to win over a conceited fool!

Every evil has its remedy, except folly.
Talk sense to a fool and he calls you foolish.
Never argue with a fool. People might not be able to tell the difference.