Same issue here.
On your iOS device you can go to Setting/iCloud//Storage/Manage Storage/iCloud Photo Library. There you will have an option to completely wipe your library.
Problem is the is a 30 days retention period before your data is actually deleted and there doesn't seem to be any way for he user to override this ... I myself have 20 days to go before I can start over again :-/
I reached out to Apple's customer support via online chat and they were not much help and basically told me to wait.
I can't believe they don't have a better solution - I didn't bother callign again as anyway I am waiting for Photo for OSX to be out of beta before I get serious about this, but maybe another

would give a better answer.