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…
Best Answer
Open System Preferences and click on App Store
Uncheck "Install app updates" (and probably "Download newly available...")