The floor of the arena should be made of hoppers so that when a player is killed their stuff is taken by the hoppers into a chest on their side. That way no one can steal in the arena. A plugin might help to make the items go to a curtain chest but it could still work without it. Each side would have hoppers set up to go to that sides chest and all the items in that chest go to that team as a prize. Or this could just be an easy safe way to play no takesys in the arena. Just a suggestion. It would be cool.
It's actually not a bad idea in theory, but it's not very realistic in practice.
It wouldn't be possible for the hoppers to place items in a chest on the losing player's side, since Minecraft would have no idea who lost or what side they came from. Players move all over the arena.
Then you'd have the issue of having people lurk by the chest and just grab the stuff as soon as it falls in..
Not a bad idea, just not practical, I'm afraid.