When I engage in a debate, I'm happy to be proven wrong. In fact, I want to be proven wrong, because that means I've learned something. There is no more noble a pursuit than the pursuit of knowledge.
That means I will actually pay attention to what you have to say, consider it and maybe alter my stance accordingly. If, at the end of a debate, both parties are actually in agreeance, then something has been achieved (and hopefully it's not just that one party bullied the other).
A troll, on the other hand, is someone who only wants to hold an opposite position in order to be inflammatory. So, they aren't interested in listening to reasoned argument and won't even concede the smallest point.
If someone is incapable of ever admitting they are wrong, they are just not worth talking to.
Of course, in the above aspect, a troll is largely indistinguishable from a zealot. But, there is no difference in my behaviour towards either.