Okay---mine is an over-preener (rescue) and she also likes to chew her ankles a bit.
Here are the things I have tried (best=first). #1 and #2 have been the best for my bird.
1. Fidget toys---"C-LINKS" HAVE BEEN A LIFE SAVER---when she starts, I try to redirect with a chain of these little links and she will twirl it around in her mouth instead. I have an umbrella cockatoo and she hasn't ever ingested the plastic when chewing-
https://www.amazon.com/Plastic-C-clips-C-links-Glider-Parrot/dp/B00SX2YGDY
2. Plain water (not as a punishment, but as a deterrent). I have found that mine doesn't enjoy the sensation of picking at wet ankles or feathers nearly as much as dry. When she is going to town, I spray the area or (in order to avoid making it look like a punishment) wet my fingers and wet the area that way.
3. Get a humidity gauge and a humidifier if necessary. Make sure you clean it religiously, as airborne bacteria and mold is BAD, but sometimes, plucking can be caused by dry skin or a nutritional imbalance. Nekton-s vitamin supplements are pretty good and the powder can be sprinkled on food- just don't overdo it, as you can OD the bird on certain vitamins. It is German but directions on my bottle are English-
4.Prescriptions---vet suggested amitryptaline (sp?) but due to liver concerns we opted for doxypine which my or may not help..
5.King's Cages Pluck-No-More---expensive...can't say it helped...used it religiously as directed and blew through a lot of money, but it might be worth a shot.
6. Preening rings---mine doesn't want anything to do with hers, but it was worth a try
7. Make sure your bird has enough stuff to keep busy (foraging toys etc)
8. Natural light and sleep can have huge impacts on hormones and anxiety. Cockatoos need a solid 12 hours of sleep and then sunlight during the day---Too much can cause overly hormonal activity but too little causes other issues.
9. Make sure you aren't accidentally giving your cockatoo the wrong idea about your relationship. Stick to petting on the head only and back off if you see any sexual behaviors (twitching eyes with a weird clicking sort of sound, mounting your hand etc). Hormonal cockatoos often pluck.