Birds by nature are afraid of hands. Before Myrtle learned to "fly" when I told her I would use an open hand with palm toward her to run her off. It is kind of like cutting a switch and then shaking it at your kid.
When I catch her coming at my head I put my hand up with fingers spread and she will just land on my shoulder. They don't like hands but love faces. On my shoulder is as close as she can get to my face and will preen me while she is there. Rambo does not fly but will climb to my shoulder and preen me.
It is all about trust. When you can read the signs of what your parrot is up to then you can respond accordingly. You have to trust him first then he will trust you. Little bites that do not bring blood can be called nibbles. Mine like to nibble. I guess if I did not trust them I would block them. I have been bit but knew what caused it and fixed the situation.
When I tell them that they are doing something they should not do they will usually stop. Myrtle has come up with "oh well" to dismiss me when I tell her something and she intends to ignore it. She has started punishing me with pinches when she does not get her way. I say "owe" and she knows she got her point across.