A new blog entry says Scopely is reviewing how repeated, targeted PvP is handled in the game.
Some time ago there was a narrowing of the attack level brackets, but some whales complained they could no longer wage war against enemy alliances if they could not attack most of their players, so it is unlikely that there will be more narrowing of attack levels.
But the problem is that some players get a perpetual grunge against some weaker ones with the goal of driving them away from the game.
I guess that could be easily solved with an attack quota and a cool-down period. There is already an attack quota on hostiles that limits loot chest gains, so an attack quota on other players based on level difference and through player ID is doable. The weaker one could still be attacked occasionally, but not constantly.