The only time I reported someone for griefing was when I was doing a Puppet's Bunker and during the second boss fight someone was intentionally trying to kill other groups with the laser beam.

They never talked in chat, but it was obvious they were doing it on purpose despite the entire Alliance trying to teach them.

We only passed it after the healers intentionally left them dead the rest of the fight.

I'm not sure if that player was ever actioned against, but I would imagine it's hard to prove someone is griefing and not just a bad player without incriminating chat messages.