With FileMaker Advanced, you can set up custom menus with the text style/size editing options removed. This should remove these options from teh file menu.
Here is another options many developers use:
They add the following auto-enter calculation to each such field and then clear the "do not replace existing values" option:
TextFormatRemove ( self )
It strips all text formats from any text entered into the field. This also, on mac systems, keeps users from pasting formatted text into the field.