It's not like they sweared on forums or watched porn. Vandalizing OSM is an effort-intensive task. You dont't do that because of lack of control. You do that because you were told to, or motivated to in any other way.
I don't see how they could do that accidentally; therefore we should seek why they did that; who knew what they were doing; and how it could happen at all.
What were their job? What were they brought in for?