Hey Hi! I got the same problem, I wanted to remove the <link> stylesheet which has nothing, only CSS classes...
Well, I found the solution after a headache. The problem comes with the plugin.
I realized that the add_action function to set the stylesheet is called in the widget constructor, so... actually if you could get the unique-id assigned to the function "wpp_print_stylesheet", the remove_action thing simply fails! (WP assigns a unique-id for the "multiple instance widgets" thing, and to catch the assigned name is not possible, only traversing the global $wp-filter and matching patterns, I have to do this)
And the problem is at the plugin 'cause this is not mean to be. The function wpp_print_stylesheet must NOT be part of the Widget class and be called within the constructor. The function could be outside the Widget class and called outside.
This happens 'cause WP (based on what I see when testing) loads all the widgets and the constructor will inevitably run. <- I think.
Other plugins I've seen, define this kind of functions outside the widget class and the actions can be removed, modified, etc. this way in the functions.php file.
I hope this could help.
By the way, nice plugin ;) I'll wait for a fix and I'll be applying the dirty solution till then.