- "Looting treasure off enemies that a party other than his own has defeated but not yet looted."
--Hard-coded out of the game.
- "Looting specific items off enemies that the player's own party defeated before the party has formally decided who should do so, or in direct contradiction to a decision that someone else should loot."
--Should have put it on "Master Loot" or not had every one click "pass." The players made their choice to pass. It's the same thing as accidentally posting something expensive in the auction house for really cheap. The person agreed to the amount when they posted. These people agreed to forfeit their claim on the items when they clicked "pass".
--Clicking "need" in the Need/Greed system is not ninjaing. Everyone agreed to this system.
- "Consistently looting all defeated enemies at the earliest possible moment, thus precluding any other party member from receiving any treasure, even when nothing special is to be had. (This is also known as "loot whoring".)"
--Don't use "Free for All". By joining a group using "Free for All" loot system, they agreed that all items are first-come-first-loot.
- "Looting items that the player cannot use for the sole purpose of selling or trading it. Sometimes this may be discovered after the fact if that same item is later found to be for sale by the player who lied about needing it. This may also occur by misuse of the in-game loot distribution system, by misleading or confusing loot rules, or simply by a lack of established loot rules. Taking advantage of these last two situations is often thought of as a mild offense and may be overlooked as carelessness or poor planning."
--This is a looting dispute/scam. The player lied so the other players would let him/her have the item. Or as stated, simply poorly defined loot rules and carelessness on the part of players in the group.
Your source even says these are not universal definitions, so stop treating them as such:
"A player exhibiting any of these behaviors might be labeled a "ninja looter", or simply a "ninja" ... Official player conduct policies also vary from game to game, as does player attitude."