You can only delete all or nothing. There is no selective photo deleting yet.
To delete everything from the device, switch Photo Stream off in iCloud settings.
To delete them from iCloud, log in at iCloud.com, click your name in the upper right corner, then click Advanced, and Reset Photo Stream. Any photos from photo stream already on your device will remain there, and any devices you connect to iCloud after will only receive new photos taken after the reset.