Can't find any setting but can't believe I can't turn off something so atypical. Anybody who has played IF for more than one sentence will know to type there!
Go to the Features tab of the game object, and tick "Show advanced scripts...". Then go to the Advanced scripts tab, and for the top script, "User interface initialisation...", click on code view, and add this line:
JS.setInterfaceString ("TypeHereLabel", "")