1) Right click on a file

2) Click "Properties"

3) On the tab that says "open with," check the radio button on the entry you wish to remove.

4) Click . . . "remove."

Note: This method is also useful for setting the defaut application for the specific file type.

Example:

Similar with many other Live CD's /DVD's, the new LMDE Live DVD does not come with Gdebi pre-installed (the GUI used to manually install Debian packages).

Even if one were to download and install Gdebi, it may not set itself as the default option to allow  one to double click on a .deb package to start the installation procedure and the default may be to initiate the archiving manager program.

This procedure will allow someone to set Gdebi as the default handler of Debian packages without having to manually search through the file system in order to find the appropriate Gbei command line.

-NR

REF: https://answers.launchpad.net/ubuntu/+source/nautilus/+question/16357

Update: This procedure is also applicable for Thunar in XFCE.