Is there also a way to UN-protect individual K2 items?
No, the protection can not be used in reverse.
My site has hundreds of items on it, with new ones being added regularly, so it's not a practical solution for us to keep adding individual item IDs to our various plans.
Best I can suggest is use REGEX and protect all ids except those listed in the REGEX. Example as follows.
option=com_k2&view=item&id=/^(?!(43|44|42)$).*$/
With the above it should match all ids except 43, 44, and 42. Replace those as needed and add more with | separating the ids.