Knowing when to debate can be tough. We get pretty messed up if our thoughts and opinions aren't respected when we're young--most parents do not, and teachers rarely do--so it can be hard to determine when a debate is going on versus when it's effectively two televisions turned towards each other.
These aren't required rules of board conduct or anything like that... but if a user flagrantly ignores these guidelines, it's a pretty good indication that they're a troll and may need to be dealt with accordingly.
Listening
It is essential to listen--really listen--to what the other person is saying. Are they making an assertion? Are they making an argument? Have they quoted the source material and accurately commented on it? Are they making any logical fallacies? Are they making any insults, whether bald-faced or veiled?
If they're making a criticism, is it specific or vague? Can you tell what specifically is being criticized or is there a lot of "it seems to be" sort of language?
If they've made a mistake and it's been pointed out, how do they respond? Do they acknowledge their error, or at least acknolwedge that there may be an error, and address the issue? Or do they ignore it, wave it away, repeat their original assertions, or completely change their story with no reference to their original comments?
It is much more important to pay attention to the form of what people are saying than the content. Don't get distracted by the content if the form is broken.
If people aren't making arguments, then if you're going to respond, point out that they aren't making any arguments or that you can't respond to the criticisms they're making.
Trusting your gut
It is also really, really important to trust your gut. When you read the other person's post, pay close attention to your experience. Is it positive or negative? Does it feel clear or muddled?
Do you feel pressured to respond? Do you feel angry or frustrated by the post? If so, you'll probably want to refrain from replying, at least initially.
Even if the feelings are vague, they're important. It might not be about the poster, of course--plenty of ideas can cause us emotional discomfort (voluntarism within the family is highly volatile but it is supported by reason and evidence)--but it is important to know what you're feeling and where it's coming from.
Don't expect that you can just do this on your own, especially if you don't have a lot of experience trusting your gut. Confer with other board members. Ask questions of their experience. Learn how to validate your experiences.
Why bother with all of this?
Maybe this seems like a waste of time and energy. "Emotions?" you may ask. "Really?"
If this is something you're skeptical about, then I get that--I'm not necessarily going to convince you in a long-ish post that this is what you should do, but bear with me and let me know what you think.
At the risk of being completely obvious, our time is limited. What we choose to do with our time means that several infinities of other things are not being done.
This means that what we're doing in the moment is valuable to us in some way, or we wouldn't do it.
Sometimes the value is a negative economic calculation: we perform some activity because the consequences of not doing so are worse (such as spending time with an abusive person because if you stopped doing so, they would attack you).
And, for what it's worth, that also applies to me writing this post 
It's important to understand what our values are, to make them as conscious as possible. If we find that our stated values do not align with our actual values, then that is an important thing to know. We've all been subjected to hypocrisy, and we're all highly sensitive to it. It's even more important to be aware of these things on a philosophy site.
But leaving this site aside, exploring and resolving conflicts between your ideals and your behavior is about self-awareness, self-care, and self-respect.
Please have respect for yourself. If you don't, you can't really have genuine respect for others, and people won't have genuine respect for you.
Another reason to ask these questions of yourself is about efficiency. Since time is limited and there is so much to be done, efficiency is something to consider.
Trolls don't just exist on the board. No, no, no, not at all.
This isn't about what goes on here, but what goes on in your life at large.
If you spend the time to understand what attracts you to a negative poster here, you start to build the skill to detect those sorts of people in your daily life.
How much of your life is drained away by trolls in your immediate vicinity? How much time ticks by dealing with people who are insulting, unclear, unpleasant, or boring?
Life is short. Take it by the balls (ovaries?). Don't waste it on the undeserving.