People that reach obscene amounts of dkp are generally the people that don't need any items. You don't reach 500k+ by spending it on every occasion, so if it ever happens that you lose an item to that person chances are it's an isolated case. He wouldn't have those amounts if he was taking every single one of your items.
Proper guilds take their looting system quite serious (dkp,epgp or any other method) so if you're losing items because you're getting trolled, you should look for a new guild.