At this time you can only delete the entire Photo Stream, not individual photos. The next iOS update, iOS 5.1, is rumored to correct this; or that is the beta's have that ability. Whether it will stay there with the final release is probable, but not guaranteed.
In the mean time, if you want to delete your entire Photo Stream, go to icloud.com in a computer browser:
On the app page, tap your Apple ID at the top right.
Tap the Advanced button
Tap Reset Photo Stream
Backout, tap Done, and you are done.
You may or may not need to turn Photo Stream OFF then ON in your iCloud settings on the iPad. I'm not sure. I don't want to erase my Photo Steam, so I didn't check.