I haven't tried this yet so this is only a suggestion...
Have you tried, like with dogtraining, to substitute what the parrot is chewing with something else?
I'd imagine when the parrot knows its not supposed to chew on something and gets attention from you to give it back (usually trying to pull it away), the parrot is just going to want it more. Learnt this in puppy classes. When they had a shoe or something they weren't supposed to have, you're supposed to offer something in exchange of the shoe, something bigger and tastier like a bone for example. They see the bone and immedietly forget the shoe and drop it because the bone is more exciting and interesting.
I think the same concept might work on a parrot. maybe im wrong. Try offer the parrot one of its favorite toys to distract him to drop whatever it is that its chewing on and take what you have to offer instead. Or maybe even a flight recall or trick to get their attention and they might drop the item.
I dont mean treating them for chewing on something they shouldnt- just distracting them or substituting the item.
