Pancuronium bromide
Normally, the brain sends chemical signals to the muscles to get them to work.  Pancuronium bromide stops these signals from getting to the muscles.  Without signals from the brain telling them to work, the muscles cannot move.  Because of this, a person who is given enough pancuronium bromide will be completely paralyzed until the drug wears off.  No muscle in their body will be able to move.