Edit: they have another compromised account that sent us a Drive document
Update and I'm not sure it'll resolve the problem since it's a Drive sharing issue, not necessarily a Gmail issue:
In Gmail Compliance, I've blocked anything containing @domain.tld (obviously substituted the real domain) of this school district. I've also went to their website and submitted a complaint via the "Contact us" page.
If there is a way to block all external domains from sharing stuff with us, is that possible via the Drive -> Sharing Settings -> Sharing Options -> ALLOWLISTED DOMAINS? If we change to the ALLOWELISTED DOMAINS, does that mean external entities not in the ALLOWLIST will be denied?
Google really has to do something. Not only are they the largest spam filtering company, they're also one of the largest proliferators of spam...