This is a less punishing alternative to hardcore. Set the initial maximum hearts that a player can have from the configuration , 20 (half) hearts by default. One (half) heart is removed upon death, until you reach zero and are banned from the server (perma death).
You can unban someone by removing him from the ipban.txt in your worlds folder. Or by using the /unban command.
I recommend you also install and preconfigure the HUD bars mod so you can see what your max amount of hearts is. Note: voxelibre doesn't seem to work with the HUD bars mod