After trading heavy blows in the previous round, Dubois dropped Lartey with a superb right hand and referee Bob Williams called off the fight.
Dubois, 21, has won all 11 of his fights, with 10 knockout victories.
"It was a fire fight, and I was still basically trying to hit him with shots," he told BT Sport.
"I let my mind go then I thought, 'You know what, I need to pick the right shots'.
"I hit him with a good shot and caught him on the point where I wanted to all night, so it was a lesson learnt."
The British Boxing Board of Control said on Wednesday that Dubois has agreed a deal for a British title fight with Joe Joyce, who was ringside in London on Saturday.
Joyce's next fight will be on the undercard of Billy Joe Saunders' match against Shefat Isufi in Stevenage on 18 May after signing with Frank Warren, who also promotes Dubois.