18 month old biting for no reason. Child is unprovoked, not out of frustration, anger, excitement. Nothing!?

I am a childcare worker of 10 years. In all my experience, I've never had an issue continue like this. I've tried every trick I know... and nothing works with this child. If someone comes within a foot of him- he grabs the child and bites them. When we tell him "NO" (VERY firmly), he has no reaction whatsoever. He is blank and does not respond to any type of reprimanding. The parents of the children who are being bit are angry. I'm angry because it looks like I can't do my job. I have no more advice to offer his mother. I've tried it ALL. Now what do I do?