Found this in another area. Hope it helps.
CharmCityCrab
In Windows, in itunes 10.x and before, what you would do is select "album view" under options, then right click on the album itself (rather than the songs), scroll to "album rating" in the menu that pops up and select "clear album rating". In Windows, in itunes 11.x and after, what you would do is select "albums" on the top bar, right click on the albums effect one at a time, go to "album rating" and then "clear album rating".
I don't know how to adjust these instructions for a Mac, because I don't own one. However, the layout of the program is probably similar enough that maybe you can use these as a rough guide and see if you can accomplish anything.