Don't get me wrong, there are some cases where it does need to be said, but I feel people are too quick to level accusations of trolldom. Furthermore, there is an assumption that because a troll thread continues, the troll was successful, but it doesn't work that way. If the people in the thread are having fun, or are getting fulfillment out of the dialog the thread generates, it's not a very successful troll. No one really fell for anything.
Outside of fair warning for others in truly abusive cases and train wrecks of threads, why bother calling 'troll'? Back in the day (in my experience), a troll was a person who was primarily interested in stirring up drama instead of sincerely contributing to conversations. The problem with trolls is not that they stir up trouble, but rather that they are disingenuous and they make it difficult for any meaningful exchanges to continue. In most cases where I see people calling 'troll', the supposed troll really hasn't disrupted the normal flow of conversation in any significant way.
These days I see people cry 'troll' at anyone who offers up an opinion that might offend, cause a bit of controversy, or seems a bit ridiculous. If you don't think a conversation is worth participating in, fine, but the person you are questioning raised actual arguments, you can't just waive their arguments away with a single pejorative. That would be fallacious.
Even if it isn't an ad hominem fallacy, unless you can prove beyond a reasonable doubt the person is a troll, it's still just a personal attack which contributes even less to the conversation than the troll probably contributes themselves. Even if the individual truly is a troll, why focus so much on their behaviour? You don't control other people; you control yourself, so why not rise above it and carry one with a bit of dignity.
In my opinion, the question shouldn't be, "Is is this person a troll?". The question should be, "Does it matter if this person is a troll?". Typically, the answer is no. As long as you can still carry on a conversation (with the troll or with other forum members) and you don't find yourself getting sucked emotionally to the point that your blood is boiling, why not just take from the exchange what you can? If that troll-accused offered even one idea that sparks an interesting thought in your head, why not turn that into a positive and explore that idea? Is it because you are afraid that it might look like you are getting taken in? That people will think you are gullible?
Even if you do get taken in by a troll, why is that such a bad thing? Internet discussions require a little faith in humanity. I know that's difficult to do, but have a little courage. Where's the joy in posting like a paranoiac incapable of believing people are sincere? What is the worst case scenario? You courteously assume the best of someone only to be proven wrong, and all the while the troll laughs maniacally in their little trol layer: "Bwahaha! You treated me with enough civility to give me a chance to prove I wasn't a total douche, but I fooled you all along. In reality I am a total d-bag. My, don't you feel embarrassed?" Honestly, is it more embarrassing to be wrong about being courteous, or is it more embarrassing to be a total tool?