To my knowledge there is not a way to "unmark" them. This has been ask a few time. I have looked through dozens of songs i did not know.. I know where you are coming from.. I have learned to copy and paste titles into search and listen to them on other sites.. I also use "My Guitar Tricks" (from homepage)and "Favorites" (top of this all pages)
For me it would be too much work to go back and clear everything. Hope this helps you out..