- invoking any other application than your preferred file manager (e.g. Nautilus), or
- none at all, and instead an error message reg. no existing associations is displayed.
In any version of Ubuntu, run this command:
sed -i '/inode\/directory/d' ~/.local/share/applications/mimeapps.list
In any version prior to Natty 11.04, additionally run this, just to be sure:
sed -i '/inode\/directory/d' ~/.local/share/applications/defaults.list
That's all! Dead simple! :D
However, as indicated above, any intended custom associations would have to be set again. So you could instead opt to open these files with the text editor and fix any possible lines manually, that is, remove a possible line under "[Removed Associations]", and elsewhere, place the entry of your desired application in front of the line of associations and remove the rest.