For a test purpose, I enabled the org-mode by
M-x org-mode in a buffer (e.g., in scratch buffer). How to disable it? Doing again an
M-x org-mode does not turn it off.
Doing again an M-x org-mode does not turn it off.
This convention is for minor-modes - Doing a M-x "minor-mode-name" again disables that minor mode. org-mode is a major mode. When you do this, emacs has no clue which major mode to go into. There must be a major mode always active.
So, rather than disable org-mode, you have to think in terms of which major mode to go into. Fundamental mode, as described in a comment is a choice as good as any.