for documentation purposes:

the action that registers the button can be removed with

remove_action( 'media_buttons', array( 'piklist_shortcode', 'media_buttons' ), 100 );

e.g. in your themes function.php file

this is true for me on piklist 0.9.9.9, the add_action statement can be found on the initialisation of Piklist_Shortcode Class, so if something changes one could look up the (possibly) changed naming to call the remove_acion