To begin with the last question: you should use an account per user, and keep your password secret. Then, only you can see your history. Of course, people with root-rights can also access your history, but they can anyway.
To clear the history, there is another button "Clear history" in the configurion. Try Konqueror
ettings->Setup Konqueror->History->Clear history (note: translated, so the real text could differ).