If they're arguing in bad faith, odds are high they're just a right-wing operative intending to sow defeatism and wedge-drive. Often they'll pretend being a concerned centrist but nowadays they feign being a leftist or tankie, which kills two birds with one stone.
With this in mind don't stop commenting, just make your strongest arguments upfront and target the bystander audience. It's the only way to act as damage-control. But don't waste your time with them, specifically.