Complex sentence: Participants must select an alias out of the Old West or having an Old West flair, which is registered with the sanctioning body that will prohibit any other shooter from using that same alias at a sanctioned event.
Split into simpler sentences: Participants must select an alias out of the Old West or having an Old West flair. The alias is registered with the sanctioning body that will prohibit any other shooter from using that same alias at a sanctioned event.