Is it possible to wipe an entire iCloud Photo Library?
Photos on the Mac makes this trivial to accomplish. Similarly with the improved web interface, this is a two step process.


Command A - select all
Delete
Select All Photos album
File - Show Recently Deleted
click the Delete All button in the top right
be patient and wait for the deletion to happen everywhere and filter down to all devices that are syncing from the cloud




Photos for OS X released with 10.10.3 has the changes needed for easy cleaning:


Open your Photos library that is the system library
File -> Show Recently Deleted
Select Delete All in the top right
Repeat as often as needed if the app is still downloading new photos from the cloud


You can also delete 1,000 photos a time using the web interface in 2019 at https://icloud.com/#photos