Eating toys like this is pretty common for cockatoos, and is generally a sign that something is missing.
More socializing and more foraging opportunities is the first thing i'd do. If you don't have a very spacey cage already, i'd consider investing in a proper sized one. (Around 2*2*1 metres, min.)
Also, make sure you check the lighting, do they have access to UV light? Are you sure your lightsources are not flickering? (Keep in mind parrot eyes have much higher refresh rate than ours, some lights which we percieve as constant flicker in their eyes.) Also, of coourse, take a look at their current diet.
And, the most obvious course of action, remove the rope and replace it with some other toys.
