there seemingly used to be a hidden preference to prevent any local storage, it required a Terminal command while Safari was closed...
defaults write com.apple.Safari WebKitLocalStorageEnabledPreferenceKey -bool false
but that doesn't work for me here.
Realistically, you'll likely find something like Glimmerblocker more useful anyway, since it can prevent any content, including the scripts that use local storage, from being loaded in the first place. Adblock or Ghostery safari extensions may also be of interest, although I've found Ghostery to block too many 'useful' comments/reviews as standard.
I installed GB and Gh today, thanks.
I had AB installed, but removed it last week. I had a problem with it, sent him an email, but he refused to do anything about it unless I created a Google account and posted the problem through that. I keep away from all things Google, and think it's a lousey way to offer support, requiring someone to setup an account on a third-party site.
AB used to make safari all kinds of slow here, say opening the bbc news homepage... seemed better at last update. If you find some page item missing, comments/review/opinions as mentioned above - look to Gh first.