To clear the scene, there is scene.dispose(), to clear the engine there is engine.dispose().
From my experience, there was an odd memory leak happening when I was using scene.dispose(), while engine.dispose() worked well at cleaning everything in memory. I could not really investigate further what was wrong in my code, but here was the thread I started at the time:
My concern as a game dev is that when I want to reload a level, or load a new level of my game, I feel that I should be using scene.dispose() but I couldn’t, because of the memory leak I was getting.