I installed Keynote 6.x recently, and decided I want to use the old version (5.3) - commonly known as iWork '09 instead.
I have deleted 6.x Keynote using Finder, but it comes back when the App Store does software updates (which also sets *.key files to open in Keynote 6.x). I can't figure out how to get the App Store to prevent Keynote from coming back...