Never argue with an idiot, they take you down to their level and beat you with experience.
This comes to mind often in online debates.
Along those lines...
Proverbs 26:4 ESV
Answer not a fool according to his folly, lest you be like him yourself.
Proverbs 18:2 ESV
A fool takes no pleasure in understanding, but only in expressing his opinion.
Proverbs 29:9 ESV
If a wise man has an argument with a fool, the fool only rages and laughs, and there is no quiet.
Proverbs 1:7 ESV
The fear of the Lord is the beginning of knowledge; fools despise wisdom and instruction.
Proverbs 18:6 ESV
A fool’s lips walk into a fight, and his mouth invites a beating