I have written previously about the fallacious arguments that conspiracy theorists often present. There's, however, one commonly used tactic that, as far as I know, has no specific name, and which I call "argument from winning debates".
This is a favorite argumentative weapon used not only by conspiracy theorists, but by all kinds of other deanialists, such as creationists, climate change denialists, holocaust denialists, AIDS denialists and so on. It requires a lot of talent and work to pull off, but it can be very effective.
It's also the reason why so many scientists simply outright refuse to debate these people: They have no chance of "winning" these kinds of "debates". The argument, thus, naturally goes: Since scientists cannot win these arguments and often outright refuse to partake in them, they clearly cannot defend their position and are in the wrong. Hence the conspiracy theory or denialist claim must be correct.
Of course that's not the reason why so many scientists "lose" these debates and refuse to participate in them. I have always liked to compare conspiracy theories (and other similar things) to magic tricks: Just because you can't explain a magic trick on the spot, when you see it, doesn't mean that there is no explanation, that it was a supernatural event. It simply means that you don't have the resources to research the trick right then and there. A few experts (usually other magicians who have tons of expertise on these tricks) might be able to spot the trick, but if the trick is new to even them, they might not have an explanation right there either.
And that's the crucial point: If you are presented with a claim (a "trick") that you have never encountered before, and the subject is very complicated, it's no surprise that you can't respond to it right at that second.
Investigating claims requires time and a lot of work. For example, if the claim is something along the lines of "the peer-reviewed paper named this-and-that published there-and-there says that ...", and the scientist has never read that paper it's impossible for him to know if
How could the debating scientist possibly know all this on the spot? It's physically impossible. The claim might be extremely easy to refute, but the scientist has no way of doing it right then and there, without actually seeing the paper and having the time to study it and its references.
So what could possibly be a laughably simple claim to refute becomes a situation where the scientist (and hence by implication science in general) appears to have no answer.
And this is just an example of one of the easiest claims to verify and to come up with an answer. Many other claims are much more laborious and require significant amounts of research and expertise.
Of course when debating to win an audience, it helps a lot when the conspiracy theorist or denialist is a good and charismatic speaker who can win the audience just by how eloquently and clearly he presents himself, in an engaging and convincing way. (Again, the parallel to magicians is quite clear.) Some of these people are really talented at this, even though they have no real expertise on the actual subject matter. Most scientists are not public speakers and do not have the experience necessary to speak eloquently to an audience. This can worsen the situation even more.
The situation would be completely different if the conspiracy theorist would send all of his claims and questions he is going to present months in advance to the scientists who are going to debate him, and then he is allowed to only present those and nothing else. This would give the scientists the time required to research the claims in question and then give the correct answers.
Of course no conspiracy theorist or denialist is going to do that. That's not how you win an audience.
It's a very common occurrence that when a skeptic offers a conspiracy theorist, denialist or creationist to have a debate through a non-real-time medium such as email, youtube videos or online journal articles, they will refuse the offer. Debates held face-to-face or via phone are ok, but never through a medium where the skeptic gets the time to do some research and respond appropriately. This is not very surprising, as the denialist/theorist would lose the upper hand and the element of surprise.
You see this "argument from winning debates" all the time. For example YouTube is full of videos with titles like "person A crushes person B in a debate", "person A wins debate against person B", and so on. Another example is Conservapedia (an infamous young-earth creationism website) which even has an article named "Creation scientists tend to win the creation vs. evolution debates". While this can, of course, work both ways, the vast majority of such arguments are made by conspiracy theorists and creationists against skeptics and scientists (in cases where they interpret, justly or not, a debate going in favor of the debater defending the conspiracy theory or creationism).
Yet, even if the debate was indeed clear "won" by one of the debaters, what does that prove? Absolutely nothing. Not about the thing being discussed anyways. (The only thing that the victory proves is that one of the debaters was either much more eloquent and prepared for the debate than the other, or that he used dirty tactics and surprise attacks to "win".)
In many cases, though, the "winner" didn't actually "win" by any possible measurement. He (or his proponents) simply claims that he won.