Frog! I have the exact same problem with my gelding! He is fabulous to tack up, load, and ride and isnt nasty at all, no ears back or viscious biting or lunging for you, but he will nibble when im not looking. Again he has never got me as i am wise to it but i do worry that others will fall for his boyish looks and charm and have a chunk taken out of their arm etc!
I too have tried everything!!! Shouting, driving him away (as an older horse would in the wild) He comes back with a sorry look on his face but then starts it up again and again and again!, also smacking on shoulder, head (i know awful! but i tried everything first!), neck, also backing him up, circling him (he thinks its a game!) and anything else that people have suggested!
He started it after he had been gelded at 13months (weird i know!), and carried it on after being turned out with 4 older colts (to bully him-didnt work!), and also doing loads of ground work with him (he did every move asked of him down to sidepass and turn on the forehand etc but still couldnt resist a nibble!). He was then broken at 3yrs hoping that that would give him something to think about.....hes fabulous when you are on his back! You can do anything to him and he is very clever and is already working in medium dressage (now 4yrs) but still on the ground he nibbles! I never let him get away with nibbling as a foal but he cant stop-its an addiction!
He comes at you with his mouth open and shakes his head...his ears do not go back and his eye stays sweet and so i started to use a tiny spray bottle with lemon juice in it and sprayed it in his mouth....it stopped him the first few times but then he would go to nibble and leap backwards knowing full well what was coming (but he is not head shy in general-you can do anything to him)
BUT i cant stop him nibbling! Some days he wont do it at all and others he'l be cronic! Its very strange!