Example: I make a post about how much I love dogs. Person 2 says cats are better. I say that cats are evil. She says that dogs are stupid.
That's where I (try to) leave it. because any more would be getting into a total online waste of time. And trust me, this has been a big step for me, it's hard to let someone else have the last word, but not as tough as the disappointment of examining my online activity over the last three days and realizing it was mostly dedicated to this one person.
Even when I'm trolling, I try to only debate with and against groups of people. It's just more good clean fun.