If your group has made a clear statement on how loot is to be handled and someone breaks the agreement, you may report them under the revised scamming policy. (Punishments include taking away the ninja'd item and potentially a suspension or ban.)
From now on, to protect yourself, request that the loot rules be clearly stated before the item you want drops. If it is not clearly stated in-game and the master looter steals something, you cannot effectively report the situation as a scam.
To explain how things were before this, May 2009 I wrote some disjointed posts about a 25-man raid my guildmates joined. A member of the guild Pure Insanity was loot master. The loot rules were clearly stated at the outset. At the end of the run, a member of my guild won a best-in-slot weapon.
The master looter claimed he couldn't loot it to the appropriate winner and promised to ticket it. Five of my guildmates stayed in the raid to make sure he didn't return to loot it for himself.
He did return, they accused him of trying to steal the item, and he used their accusations as justification to steal the item. He and his guildmates taunted my guildmates. My GL talked to their GL, who was polite but did nothing. My guildmates then put in tickets, but the GMs didn't do anything but offer platititudes. We had a report from a more sympathetic Pure Insanity member that when a GM spoke to the ninja, the GM asked if he'd been the master looter and then said it was the master looter's choice to do what he wanted with the items that dropped.
My guild was dissatisfied but unable to pursue any other form of justice. The people who'd been on that run cut off all ties with Pure Insanity, and the rest of us avoid them as common sense.
Thus, my guild is quite happy about this change. The same tickets that the GMs couldn't help with would be valid now.
This change can also be useful for members of guilds with untrustworthy raid leaders. Simply whisper your leader and ask them to repeat the guild's loot rules before you start a raid (easy enough with a macro).
Good raiding, everyone!