Go to iCloud.com/Photos using a computer browser to delete them.
The only way I am aware of is to do a manual delete at iCloud.com/Photos. Hold down the command while selecting the photos. Then click the Trash Can. You can also try selecting a moment, then click the Share button, and then cancelling the Share. The photos should remain selected.
If you have access to a Mac computer running 10.10.3, open Photos, go to Photos/Preferences/iCloud and set up syncing. Then go to System Preferences/iCloud, sign in, and check Photos syncing. Once all the Photos have synced to your computer, quit Photos. You should now have a complete compilation of your photos (like a Master Library). Reopen Photos with the option/alt key held down and create a new system Library. You can now delete photos from your that Library while maintaining a master copy on the computer. Turn off iCloud syncing when reverting to the Master Library.
Photos - Use multiple libraries in Photos