Ah! I found the solution myself. The editor is called EditArea (
http://www.cdolivet.com/editarea/editarea/docs/configuration.html), and it's in the /include/editarea folder. The file wb_wrapper_edit_are
a.php has the settings for WB. By adding 2 lines to the file, you can set the default state for the editor to OFF. So default is just a clean and fast text area, and with the Advanced toggle you can invoke the editor if you want to. Actually I think that should be the default setting for next WB versions.
So if you want to turn the editor off by default, change
$min_height=300,
$toolbar='default'
to
$min_height=300,
$display='later',
$toolbar='default'
and
toolbar: $toolbar',
language: '$language'
to
toolbar: $toolbar',
display: $display',
language: '$language'