how can I erase some of the photos in my i phone

Import the photos from the shared albums in the Photos Library on your Mac. (ctrl-click the photo in the shared album and select Import from the contextual menu.). Once they images have been imported into the Photos Library on your mAc and appear in the Moments, you can delete them from the Shared album.

Make a current backup of your Mac before you start a massive delete, just to be on the safe side.

Are you using iCloud Photo sharing or iCloud Photo Library?

With ICPS you simply import the photos form the shared albums to Photos then they can be deleted form the shared albums

With ICPL there is not way - all changes made on one device are made on all devices including additions, deletions and eidts

LN