Not sure if I understand your question, but you're telling that you have a single datastore built on top of 6 lun ? Each lun as a extent for the datastore ? If yes, the supported/correct way to remove one of this lun for the datastore is migrate all virtual machines from the datastore, delete the datastore and re-create the datastore only with the luns you want.
It is not possible to remove an extent from an existing datastore. Once an extent is added, it is not possible to remove it without compromising the data residing on the extent.---
Senior Infrastructure Specialist