I got the best solution for you marre but you will need someone which is really good with scripts and so.
Heres the idea i got for you.
If player1 attacks player2 within the range of 8-40 and the damage is > (0) then block all damage.
Sure this will prevent the mass killing on the range of Level 8-40.
But wont solve the problem with blocking in wars. So i thought of something i've seen in another Server which is...
If a player within the range of 8-40 walks into a player higher then 40 then pass through them.
Its just an idea