OK, I found it - it's hidden in the FILE menu, an entry that only appears in the voxel editor. Still, a clearly visible button would be great.
There is a button to clear the scene editor where you import STL (or others) but there is no way to clear the voxelized scene?
Moreover, when you don't clear it and voxelize another scene (or the same again) it'll overlay them - if they're identical you don't even know it's twice, but it will calculate twice the time and filament to print.
A button to clear the whole scene would be nice.
Or being able to mark an item with the mouse and delete it by pressing DEL on the keyboard. (Which works in the scene editor).