In Family Tree Builder, in Tools> Advanced Privacy Settings, you can choose which people you would like to privatize so that they will be published as Unknown. To privatize all living people, choose Add person, and then on the small magnifying glass on the top right to Search. Then select in Facts - Death fact, and Does not Exist.
That works...but for the average user...just clicking a check box to block uploading living people information is easier than building a query. Just a suggestion to make it easier for new users. Just adding a few quick privacy options to the publishing screen would help people protect their data that they do not want to upload.