You can always curb the behaviour by correcting it. Each time they beck you, take your index finger and give them a big, strong poke right on their head. Usually if my chickens bite, I will pinch the back of their neck ( with other hand on their back so they can't struggle and hurt themselves ), and push their head onto the ground and hold them until they relax. This is how chickens tell other chickens "NO!" in chicken language, and it's worked to convey my dominance to my flock, and I've got two roosters and three hens! No problems from anyone in ages!! My roosters don't even crow, because they know I'm the boss and I'm keeping everyone nice and safe, and they've all got everything they would ever need ( though I am trying to find them toys because I fear my being sick the past few weeks has made them dreadfully bored ).
Just don't EVER aim to HURT your animal. When doing this you NEVER want to make your chicken scream ( though the first few times they will just because it's a startling experience to suddenly be dominated by the person they always thought was a big push-over - this is not a painful scream, and a painful scream is MUCH different ), or scare them. Your ultimate goal is to feel the muscles in the body relax, and then release them, teaching them "if you are relaxed and calm, then you will be free and happy."