It doesn't matter if it is a spy for the government, or an employee of a corporation, or simply a sadist and sociopath like the vast majority of trolls.
The fact is that all trolls use the same "tools". By recognizing their methods we can take away their power to disrupt reasonable debate.
First I want to state my qualifications. I've been debating online since years before the creation of Netscape Navigator. Back then we used to debate on usenet.
All the debating methods that I'm listing below I've fallen for countless times. While I'm using several other sources for this list, and main source is from my own experience. Most trolls tend to stick with just a couple favorite methods.
There are two things to keep in mind about trolls:
1) They don't want to talk about the topic at hand. They want to talk about you, or themselves.
2) It's never too late to call out a troll. Even after you are five or more responses into the thread when you finally realized that you've been suckered, you may be incline to stick with it out of some misplaced pride or anger. Don't. You can't win debating a troll. All you can do is lower yourself to their level.
Tools for trolls
#1) Name-calling and ridicule: That doesn't mean just calling people names. The objective is to goad other people in the debate to start name-calling. That means use baseless caricatures and stereotypes, generally directed at the author, to get an emotional response. Appeal to people's prejudices and fears.
#2) Play Dumb: You are immune to facts, and all news sources are suspect. Even common sense logic is to be questioned.
#3) Demand complete and fool-proof solutions to the problems: How can you bother other people with these problems unless you also have the solutions? And not just any solutions, but solutions that have been scientifically tested and work. What? You aren't an expert in the field? Then why should we care what you say? This method is most effective when the proof you demand is impossible.
#4) Build and Destroy a Strawman: Create the elements of your opponents arguments, as you define them, and then tear them down. [This is one of my favorites]
#5) When in doubt, declare victory!: You have no idea what the debate is actually about, and all your other debating methods have failed. So why no celebrate? After all, it's all about you.
#6) It's hopeless, so why try?: Nothing can ever change. We are all doomed. The powers-that-be will always win. [This appears to be a recent favorite on the internet, and obviously the most useful for maintaining the status quo]
#7) It's old news: Never mind that the newspapers seem to care for some reason, anyone who's been paying attention had to know that this has been going on forever. [This is related to #6 and #4]
#8) Suggest extreme solutions: The government did something wrong? Then let's get rid of the government! Kids are having trouble in schools? Then public schools must go!
#9) Wax indignant: (aka "How dare you!") Obviously if someone said something you didn't like then you should take it personally because it was meant to offend you.
#10) Shout down others: This is accomplished by posting a bunch of messages before your opponent has a chance to respond. You can do this because you have more time on your hands than most people.
#11) Army of sock puppets: This method depends on your resources. It's extremely effective. If you are a government agency or a corporation, then this method may have the most bang for the buck.
#12) Hit and Run: You won't stop debate with this method, but you sure can disrupt it. This method is most effective when used with #1.
#13) Question motives: This is an oldie but a goodie. Obviously if someone wants to call attention to something then they have something to hide, and it is your duty to expose it. What sort of partisan agenda is he/she pushing anyway?
#14) Invoke Authority: You are an expert on the subject, and these accusations are nothing more than wild rumors and conspiracy theories.
#15) Demand that they solve the crime: Why should we get excited over this unless we know every single detail? After all, this is an extremely complex situation and difficult to understand.
#16) Come half-clean: By admitting to unimportant facts then you can appear to be completely honest. [This is a last-resort method]
#17) Make shit up: Another oldie-but-goodie. If you demand your opponent prove things wrong then he/she isn't able to focus on the topic at hand. It doesn't matter if your "facts" are irrelevant to the topic. Plus, if they are unable or unwilling to spend the time disproving your BS, you can say that you have "proven" their arguments to be false and go declare victory.
#18) Make even the slightest mistake something important: If you opponent makes a typo or isn't clear about something, then obviously they have no credibility. If the newspaper they are sourcing once made a mistake, or is from another country, then they can't be trusted.
There probably is more methods that I am forgetting, but these are the primary ones. If you think of others, please put them in the comments so that I may add them to the list.
9:41 PM PT: Several people have pointed out that many of these methods are a) commonly used tools to "blow up a troll", b) that can be used for non-troll activities, and c) sometimes get used by good people on an off-day.
This is all true.
I'm not trying to say that if someone uses any of these methods that they are automatically an evil troll. I'm saying that evil trolls almost always use these methods.
One does not automatically mean the other.
The purpose of my listing these methods is so that if you find yourself in a debate with someone who is quickly pissing you off that you should take a step back and think if you recognize his/her debating methods that I listed above as troll-like. If the answer is "yes", then you are safe to call them out as a troll and walk away.
This is a much better outcome then uselessly getting down in the mud with the troll where no good can come from it.
9:43 PM PT: There are two other suggestions in the comments:
#19) Concern Troll: This is more of a political tactic than a debating one, although it can be used for both.
#20) Blog Nanny Patrol: I'm going to admit that I'm not exactly sure what this is.