Can I safely delete from my iphone photos shared by friends?

I would like to free some space on my iPhone 4S by deleting my local copies of photos from a friend's photo stream. Can I do that without affecting my friend's originals and without signing out from the stream? Ideally, I would like to do that selectively, i.e. delete some of the photos and keep others. Also, I would like to continue to receive my friend's newer pictures.