The official SD Card Formatter recommended for installing NOOBS is unable to format an SD card with a previous NOOBS installation (despite its impressive and slightly scary version number of 4.0.0.0). You can, however, use Windows’ own diskpart.exe utility to wipe the SD card:

DISKPART> list disk

Find the disk number N of the SD card.

DISKPART> select disk N
DISKPART> detail disk

Verify the disk size to be absolute sure you’ve selected the right disk!

DISKPART> clean
DISKPART> exit

Now the SD Card Formatter will accept the card. Maybe version 5.0.0.0 will be able  to format SD cards even with existing partitions :-)