Green’s Dictionary of Slang

stop what you are doing!

1956		‘Charles Raven’ Und. Nights 103: Barney from the door hissed ‘Stoppo!’ and they had to dash out on to the balcony.
1959		‘The Pickpocket’ in Encounter n.d. in Norman Norman’s London (1969) 66: Nitto, nark it, stoppo, / That’s wot the whizz mob are saying, / Take yer fork from his outer, he’s piping, / If yer banged then yer in fer a striping.