You have let the training get way out of hand. It should never have gotten to this stage. Pet parrots need just as much training to be a housepet as dogs do.
You now have to make biting things other than food an unpleasant experience for it. Make various potions with vile tasting stuff such as a bit peanut butter mixed with the same amount of mustard. Smear a bit over the stick or finger when you tell it to "step up" and the biting will soon subside. You have to keep it up with different tastes and object.
It is not going to be easy and you have to persevere. Any bad habit with any pet has to be nipped in the bud asap. The longer it goes on, the harder it is to quell the bad behaviour.