It's pointless to try to reason with a stubborn person.
Literal
Stubborn person [with-と] properly discuss-even useless.
偏屈な奴 is 'a stubborn/cranky person' — 奴 (やつ) is rough, informal language for 'that guy/person.' まともに means 'properly/seriously.' The ~ても無駄だ pattern means 'even if you do X, it's useless.' The sentence carries a resigned, experienced tone.