Hi Fernandor,
The "autofill" is Composer's way of keeping track of how much time in each measure is filled with notes (which you enter) and rests (which it enters). There is currently no way to turn off this feature, but it may help to learn how the Beat Ruler works.
If you have not already, I would suggest reviewing the Tutorial Videos on Note Entry at
http://www.notation.com/vb-forum/forumdisplay.php?f=2858 The "Note Entry" videos actually show you a few different methods that are available for entering notes. Hopefully the videos will help with understanding how the different methods work, making it easier for you to enter your notation.
ttfn,
Sherry