defaults write com.apple.dashboard mcx-disabled -boolean YES
Next, kill the currently running Dashboard by killing the Dock (the Dock will reload itself, don’t worry):
That’s it, now Dashboard is completely disabled. Hit F12 and nothing will happen. So what if you had a change of heart and want Dashboard back? Well turning it back on is just as easy.
Enabling Dashboard– type or paste the following exactly into the Terminal window:
defaults write com.apple.dashboard mcx-disabled -boolean NO
Again, kill the Dock, which will reload the now activated Dashboard: