Out of pure luck I think I stumbled on the ideal solution to delete all photos at once from my iCloud photos library (I assume your question is pertaining to iCloud.
My issue, along with everyone else is that Apple removed the ability to select and delete all photos from iCloud all at once (obviously because by doing so they try to force users to buy addition iCloud storage. SCAM!!). After 2 hours of searching (again) I was starting to get really ****** off.
So here is how I managed to delete all iCloud photos on my iMac, which is one of the devices that I have synced to iCloud, which is necessary for this to work. (I'm not sure if this would also work from the iPhone Photos app as they keep changing the functionality of that as well). Also, haven't used a PC in 10 years so not sure if/how this would work on one.
1. Make sure that your iMac is synced to iCloud.
2. When step 1 is met, open the Photos app on your iMac and select All Photos (if you want to delete all, of course).
3. Press Shift + A to select all the photos at THE SAME TIME! :-)
4. Right click on the screen (I think anywhere is works) and select "Delete Photos (XXX items)".
5. Now if you log into your iCloud account (icloud.com) you will see it is no EMPTY!! All photos have been removed from there and if you check the Photos app in all your other synced devices (iPhone, iPad, etc.) you will see all the photos have been removed from them as well!!
Such a simple yet completely counterintuitive solution to deleting photos from the iCloud library when it gets full. I hope this helps people save the hours of time I spent trying to figure this out.