It's not at all hard to pinpoint exactly which loot roll it is that you got ninja'd because the raid chat will most definitely call you out in the same loud volume. That is unless you leave the group with unrolled items, which is then, those items. As a person who has reported for ninja looting, I can tell you that it is incredibly difficult to gather up enough evidence to actually get someone banned. They require a message in trade chat about recruiting to the raid, a screenshot of when you get into the raid, a screenshot of when the raid is full and a screenshot of the item being ninja looted.
Providing the evidence that has been received to get the person banned is not only unnecessary but also harmful to the community as it can, and likely will, open doors to witch-hunting the people that did the report. Warmane doesn't keep chat logs, so the only "evidence" they can show in return are screenshots that must be in full screen, which includes the name of the person taking the screenshot.
Anyhow, I've been here for 3 years and haven't been in-game banned once. If you got banned, there is a reason.