Graphical notes can be deleted using the Comments catalogue which will also remove them from the document.

Alternatively:

  1. Select the Hand tool (Hand tool icon) on the main toolbar
    or the Stamp tool (Stamp tool icon) on the Commenting toolbar (View->Toolbars->Commenting)
  2. Right-click on the stamp and choose Properties...
    Change the Appearance of the stamp as well as it’s Opacity - handy if you want the page's contents to show through. The Appearance drop-down menu will only show built-in stamps (user defined stamps are not included)