Disable dictation from command line
If you are talking about not having the message box, shown below, popping up again when pressing the e.g., Fn key twice, then use the following defaults command in  Terminal:

defaults write com.apple.HIToolbox AppleDictationAutoEnable -int 1


This changes the Shortcut: setting from e.g. Press Fn (Function) Key Twice to Off in System Preferences > Keyboard > Dictation, and would be the same setting that gets set if clicking the Don't Ask Again button.