go to icloud.com
log in click on your name
then click advance
then click reset photo stream
from apple site:
After resetting your Photo Stream, you can also remove any photos that have been downloaded to your device or computers. To do this, follow the directions below:
On your iOS device, you can remove photos from the Photo Stream view by turning off Photo Stream and then optionally turning it back on. You can do this at Settings > iCloud > Photo Stream.
If you added any Photo Stream photos to other albums or the Camera Roll on your iOS device, you can locate and delete these photos manually, if desired.