You have to log into iCloud.com from your desktop (mobile won't work right), click settings in upper right hand corner by clicking your name, then Advanced, and there you can reset it. You'll have to toggle photo stream on/off on your idevices afterwards to finish resetting
Unfortunately, no. It's far from a prefect system...yet...but, still a good start. The photos will fall off the stream after 30 days, or when you reach a 1000 photos or so, they start to get replaced by newer ones.