"Hold it right there!" (4)
(Other definitions for whoa that I've seen before include "Said to stop a horse" , "Stop, Dobbin!" , "horsy expression?" , "Hold your horses!" , "Cry for horses to stop" .)
(Other definitions for halt that I've seen before include "Bring to a stop; lame" , "Stop, small station" , "game" , "Small railway station" , "limping" .)