How do you defend against hackers using these to attack your SaaS server? You can't block a single IP because they are dynamic. You can't block the address range because some IPs are legitimate users.
This is a broad question. There are multiple ways to defend, besides basic security like firewalls. Blocking nefarious IPs or domains only gets you so far, as something will slip through the cracks. Try reading this: https://opsec101.org/