[Xrystal]

You could try calling this function on each of the Arena Frames. This is called when you adjust the CVAR variable

function ArenaEnemyFrames_Disable(self)
  self.show = false;
  ArenaEnemyFrames_UpdateVisible();
end

This is the function that calls it in Blizzards addon.

function ArenaEnemyFrames_CheckEffectiveEnableState(self, cvarUpdate)
  if (C_PvP.IsInBrawl()) then
    ArenaEnemyFrames_Disable(self);
  else
    if ( GetCVarBool("showArenaEnemyFrames") or cvarUpdate ) then
      ArenaEnemyFrames_Enable(self);
    else
      ArenaEnemyFrames_Disable(self);
    end
  end
end

Which is called by the Event Manager function

if ( (event == "CVAR_UPDATE") and (arg1 == "SHOW_ARENA_ENEMY_FRAMES_TEXT") ) then
    ArenaEnemyFrames_CheckEffectiveEnableState(self, arg2 == "1");

Your code should work I believe but you will need to have an frame to monitor the event ADDON_LOADED so that once you Load Blizzards addon it will trigger the event and its information should be available to use and adjust. I thought the SetCVar would trigger the CVAR_UPDATE event but it is looking for a different CVAR value to what you are setting which could explain why it isn't doing anything noticable. Maybe changing

SetCVar("showArenaEnemyFrames", 0)

to

SetCVar("SHOW_ARENA_ENEMY_FRAMES_TEXT", 0)

and see how that works, then try the ADDON_LOADED steps I suggested.