I also followed another person's strategy to deter biting. He said to not react when they bite, but if they nudge you with their beak (like pushing you away) to immediately leave them alone and walk away. Supposedly that lets them know that biting won't make you stop but if they bump your hand with their beak, that's the sign to go away.
I did that with my birds and it worked like a charm. They haven't bitten me in a few months and when they want to be left alone, they just push my hand away with their beak. And I believe they are past the bluffing, so they are more sweet in general, and Kashi who was a real terror is wanting to spend more time with me again.
Here's a pic of the gruesome twosome after a bath

