How to disable file associations to an app before/after installing?
lsregister is the command to deal with Launch Services. To unregister an app use the -u option and the path to the app bundle. lsregister is not in the PATH - you have to use the full path to the executable:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -u /Applications/MediaPlayer.app