Should there be Atheist groups?

[Tiberius]

For example, is there a word for the disbelief in ghosts, or aliens or salmons orbiting Jupiter etc?

If there were a word specific to such a believer I suppose there would be a word for someone who disbelieves it. In each case though, you only have the "belief" of such things. So if you called someone who believed in salmons orbiting Jupiter a "Salmonite" then you would have the people who call themselves the opposite (whatever it may be).