the wp_options renaming works for me.
@arbe-vanbeek: not a very useful solution, but like you I wanted the styelsheet off. So I edited the plugin (you can do that via Plugin -> Editor) and commented out line 17:
// add_action( 'wp_enqueue_scripts', 'category_posts_widget_styles' );
(added // at the start of the line)
I also added, on line 3, on the plugin’s description, a note to myself — so that if I see an upgrade for the plugin, I’ll remember to comment out the stylesheet again:
Description: (MODIFIED: REMOVED STYLESHEET) Adds a widget that shows the most recent posts from a single category.
I hope in future versions, though, the inclusion of the stylesheet will be an OPTION.