Hi shiny;
Good question, hun. And a grim thing to do with, we've had similar problems. It depends on "why" a bit, I think. Is it:
- frustration
- sensory
- attention/communication
We have a thing called a "learning disabilities nurse" who visits and he is actually quite helpful with stuff like this. And admits when there is sod all he can do which is also refreshing.
DD1 eats her hair (rather than pull it out) so it is all very tightly plaited and then gathered up into a pony tail at the back of her head which is fixed with lots of the strongest elastic bobbles I can find. (I surreptitiously ping them every time I see one in a shop). That will never be allowed to be long enough to eat iyswim. If she works out how to bust out of that, I will just cut it.
She used to chew her hands till they bled. We had a bunch of strategies - give her something better to chew (teething ring type things - we found the best came from France ... of course ... but I suspect it's individual to the child anyway) redirect her in a positive way (ie never give her attention/negative reinforcement for chewing her hands, say "where's your CHEW, DD1? Oh, good girl chewing your CHEW") and then when you can't get at her, forcibly restrain. So in her car seat, her hands were tied down, I'm afraid. And at night she wore those excema pyjamas with integral mitts and ties round the wrist. My main worry was that she chewed the joints of her thumb a lot and I feared she'd get arthritis and lose mobility: only one hand works anyway and if she loses the opposable thumb grip in the other, she's stuffed.
We never really worked out why she did it, so we tried everything at once, really. And then one day she just stopped. I think it was sensory myself (though OT says otherwise, nasty political animal that she is) and she eventually decided that sucking her thumb was a better sensory buzz than chewing it.
Don't know if any of that helps at all, beside knowing there are others in the same boat.