I don't believe this is directly possible - and judging by the API, I believe this is not the case. You could probably simulate it by registering a listener that cleared the text box after every keypress.
However, are you sure that you want to do this? Personally I'd be very upset if I was using a file selector that was crippled in this way, especially if I'd copied a file path to the clipboard from somewhere and was forced to manually descend through a bunch of directories, amongst other situations.
Is there a particular reason you want to cripple the user interface? Because I can't see any legitimate reason to do so...