I watched a program last night about 9/11 conspiracies, namely the Twin Towers were not bought down by aircraft crashing into them but it was a controlled demolition carried out by conspiractors.
The conspiracy theorists claim that a fire could not have weaken the metal frame of the twin towers enough to make them collapse but explosives or a new and secret type of incendiary material was used instead.
Experiments were done to test these ideas and in both cases, the conspiracy theorists were proven wrong. They did not accept the emperical evidence from the experiments but they did no provide any counter evidence of their own.
It seems to me that if you want to believe in something enough, then any evidence to the contrary would not deflect people from those beliefs but instead, other objections which are also unproven would be put up.
A bit like Creationism.