First off..dont give your donkey any treats. Once a donkey learns to bite it can be hard to break them of this. Your best way of stopping this kind of behavior is by giving the donkey a loud NO or STOP, and when you say it ..you need to mean it.It has to sound harsh....like your ten times your size.
You can try Susans way, but I wouldnt recommend it..pinching and slapping a donkey can create more problems then just his biting.
OH! He can become headshy and afraid of you. Also, alot of times with aggressive bitters, it can happen so fast you wont have time to grab a lip and twist...(pinching and twisting has to be done IMMEDIATELY, as he is trying to bite you.. not a minute after he has bitten you, or he will turn the pinching into a game, and more problems!)
OH! I have delt with rescue donkey who were fear biters/kickers, all of them turned into sweet loving animals, with no harsh treatment of any kind. BUT..remember NO TREATS for biters. Donkeys have a remarkable memory. I have one here right now that I am working with on his manners and biting, we're coming along great!
2 months ago, you couldnt enter his stall...now, I can do anything, for as long as I want. He learned trust, no kicking, and no biting all at the same time, along with manners/space. Corinne