The core of all UFO conspiracy theories is that extra-terrestrial aliens, working with secret government agencies, are somehow hiding the truth from us. These come in several packages. Now, these various theories often overlap, but I'll try to keep them destinct.
Men in Black. Now, don't get this confused with the wonderful film written by the gifted Ed Solomon, nor the comics upon which the film was loosely based. The conspiracy is that aliens, the progeny of aliens cross-bred with humans and a secret world-wide agency are directly involved in intimidating ufo true believers, abductees and experts into keeping silent. They are also, like they are in the films, actively involved in cleaning up after ufo encounters.
Now, the fact that the only evidence of these men in black is purely anecdotal, should be enough to squash this conspiracy theory, but add to that they in the many tales about them, they often have conflicting goals. They have access to widely varying technologies form none to space-time warps. They are seemingly invisible to most people, they pass as ordinary people to nearly everyone, but they are easily recognized by the ufologists, because "they are the only ones who are not fooled by the conspiracy."
Mars Attacks. Aliens are seeking to take over the earth. They are doing this mainly by infiltrating industry, media and the scientific community with cloned humans who look like humans, but who are really genetically modified aliens built to resemble us.
If you replace the word alien with the word Jew, you get the conspiracy theories of the 19th and 20th centuries which led to pogroms, genocide and hate crimes uncountable. It's terribly convenient that these infiltrators looks so much like us that we can't tell them apart from humans except by their "inhuman actions and motivations."
Independence Day. Aliens have crashed on earth and we are holding their technology, their corpses, their space craft, their messages, in hidden military bunkers. We have reverse engineered such advances as the radio, transistors, military drones, superconductors, computers, not to mention cold fusion, water-powered automobiles and anti-gravity which they aren't releasing yet.
The problem with these theories is that they are patently unstable. They posit the existence of a huge secret that no one has been able to reveal, and yet they require a horrific number of people to maintain. Now, if we have learned nothing else in the last 50 years, then we should know that the investigative media will stop at nothing to find a good story, and this is the best story of all. But, even though the President of the United States cannot get oral sex from an intern in the privacy of his Oval Office without the whole world knowing, should tell us that even the most powerful man in the world isn't invulnerable to secrecies revealed.
Conspiracy Theory. The government is squelching the release of any ufo information that might be damaging. They ban books, they wipe out internet sites with super viruses. They are engaged in assassinating ufo experts, and if they are unable to do that, they assassinate their reputations.
Well, I can't imagine that anyone thinks books are banned in the United States for any reason. Schools or libraries may ban books by choosing to not place them in curriculum or on shelves, and publishers may choose not to publish something, but this hardly constitutes a conspiracy. Take a look at the ufo shelf of any bookstore and you can find any piece of information about ufos you may desire. And the internet has a gazillion sites that have been online for ages that purport to tell us everything from the above conspiracies, complete with photographs, interviews and utube video. Nope, there is no effort to hide the truth from us. We have to wade through this detritus as it is.
Signs. The government has engaged in disinformation in order to hide the truth from us, ridiculing experts and true believers, silencing them when possible and eliminating them when not. Now, this may or may not be true, but the overwhelming evidence is that the government has not been effective in achieving this, and that any attempts to actually assassinate these ufo star experts was about as successful as the attempts to kill Castro.