You can only delete them all at the moment. Point browser to www.icloud.com and log in. Delete them from the cloud. Then, on each device, turn off photo sync to wipe them from the device, then turn photo sync back on.
First, save all photo's you want to keep in the stream to your library in iPhoto > go to icloud.com > log in and click your name in the top right > Advanced > Reset Photo Stream > then turn photo stream off and on again on all your devices > to add the photos you do want, drag them from your library into Photo Stream using iPhoto again.
Hopefully Apple will implement a simple way to delete just the photo's you don't want in a future update.
You can only delete them all at the moment. Point browser to www.icloud.com and log in. Delete them from the cloud. Then, on each device, turn off photo sync to wipe them from the device, then turn photo sync back on.
I don't see photostream on icloud.com. What I have done in the past is to delete photos from photostream on my computer and disable photostream on my iphone. Disabling on my iphone asks me if I want to delete them and I say yes. This worked for me.
But I don't see photostream on icloud.com.
EDIT: Thanks mopatops! You posted shortly before I did!