How can I delete my photos off iCloud permanently?

If you sign in with Apple ID and password at www.icloud.com and delete photos they are moved in recently deleted folder , tap recently deleted folder to select all photos hold shift key and tap on last photo , click on delete all ( number of photos ) a pop up window appears you can delete them permanently from the iCloud server .

Delete them and delete them again from the recently deleted album.

If you are syncing your devices with iCloud, they should also be removed from it.