How is griefing not reportable?

This guy ***** joins our game at the end, just as we were running to the extraction with our samples. He gets in to a mech and kills us all before we can get into the extraction shuttle. I wasnt able to kick him in time and we hadnt any reinforcements so the mission ends with 0 survivors and 0 samples.

We were all furious and we believe this person and people who behave like him must be banned, if not banned at least should get a punishment of some kind.

The question is how is this not a reportable offense? What does a person needs to do before he gets banned? Team killing, griefing the mission, the chat all of these are not reportable.