Hate to nitpick but there is no such thing as Spiritbending. It is only canonically known as Energybending. Spiritbending is a fanon name.
Also, I think that the 6th chakra is very important to Amon's "energybending". I think that he closes a bender's 6th chakra and disrupts the flow of chi in the body (which is why they all collapse right after he does it) and since the obstruction of light is darkness/deceit, the lie that their bending is gone permanently prevents them from opening the chakra and regaining their bending.