Unable to set quota to zero

I saw a posting on how to limit users to only uploading to shared folders. The solution was to put the quota to zero.

When I go to the quota settings when I put 0 or 0 bytes it gives me an error of invalid quota size.

1) How can I set the quota to zero?
2) Or how can I limit uploads to shared folders only?

in 10.x you could use the guest app to accomplish this. Which server version are you trying?
If 10.x then please file an issue in github in core about this, might indeed be a bug.