I have recently started working as a BA, At my work place I constantly deal with invalid requirements, for once and all how to put an end to getting these requirements, I am not sure what approach to take. Any suggestions would be appreciated.  

Where do you maintain these requirements?

I capture the requirements in JIRA. If invalidated - I make a note against them ("As per John Doe, this is not valid because..."). If similar requirement is raised, then I add reference to the previously invalidated requirement and cancel the new 'invalid' requirement.