In the macaw forum there's a sticky thread "big beak-o-phones guide to macaw beaks" which is geared toward those scary big beaks, but lots of the info will apply to your bird. I can tell you one thing that worked with me and a blue front....he bit me goood a couple times and I was careful not to react. Than one day I offered him a cheezit cracker...he leaned for it then turned his head to nip at my hand....I was able to pull back before he nailed me...I looked at him, he glared at me...I held eye contact and ate the cracker myself. Saying um yum yum. It was nasty stale, btw. This is where you learn to hone your acting chops.
His eyes grew wide, his beak fell open, he couldn't believe I was eating HIS cracker...he danced back and forth....he looked so surprised and shocked....I ate the whole thing, gave him a look, took another cracker out, held it in my hand and looked at him, he looked at me, I held it out and he took it nice as pie and never tried to pull that stunt again.
Point being...he was smart. I had his attention and he had mine, and we communicated to each other. He knew exactly what he had done, and what the consequence was, and he knew what would happen if he cooperated. It's the same with your guy. You have to pay attention to see what's happening when he gives you snip. Is he just over-excited? Did you do something he didn't want you to do, or try to make him do something he didn't want to do? He isn't going to understand "no"... so let the moment and the circumstance dictate how you respond, to let him know that isn't cool and will have consequences.