How to remove the default application for opening a file?
Use RCDefaultApp. With it, you can set the file association to &lt;disabled&gt;
Another option is to use the command line and lsregister. lsregister is located in /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/ To see all the available commands, run /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister. See this article for more information.