2015-02-08 OS X
I know nobody who finds OS X Dashboard as an useful tool. There is a easy way to permanently turn in off by typing just few lines in terminal.
Open OS X Terminal and paste the following line:
defaults write com.apple.dashboard mcx-disabled -boolean true
Hit Enter and then type:
After the Dock will be reloaded, your dashboard should be disabled and no more accessible. Keep in mind that if you want to undo those changes, just repeat above steps but change “true” to “false” and your Dashboard will appear again.