Well the 6th chakra is blocked by illusion to be specific, so the illusion that they lost their bending permanently blocks it from being reopened.
If that is true, do you still need a special chakra blocking ability?
I like the theory, but is Amon special or could every equalist put on the mask and poke a bender's forehead and the bender's believe that their bending is taken away would be enough to block the chakra?
Well Amon is much more than a man in a mask. His personality projects so much confidence, so much power, it would be hard for anyone who has seen him in person to not believe he could do anything. Look at how he instills so much raw fear into Korra. That there sets up the illusion so when Amon physically blocks the chi paths and closes the chakra it doesn't reopen.