How to disable default alt-left / alt-right keyboard shortcuts for Numbers.app
You can edit the plist directly:

defaults write com.apple.iWork.Numbers NSUserKeyEquivalents '{"Add Column Before"="\0";"Add Column After"="\0";}'

It overwrites custom shortcuts set in System Preferences, but you could also edit the plist in a text editor after converting it to XML:

plutil -convert xml1 ~/Library/Preferences/com.apple.iWork.Numbers.plist