Disk Management and DISKPART don't let you delete partitions on removable media. But you can use DISKPART's clean command to nuke it and start over. Here is a series of commands that worked for me:
DISKPART> list disk
Disk ### Status Size Free Dyn Gpt
-------- ---------- ------- ------- --- ---
Disk 0 Online 40 GB 0 B
Disk 1 Online 7538 MB 0 B
DISKPART> select disk 1
Disk 1 is now the selected disk.
DISKPART> list part
Partition ### Type Size Offset
------------- ---------------- ------- -------
Partition 1 Primary 7 GB 1024 KB
DISKPART> clean
DiskPart succeeded in cleaning the disk.
DISKPART> create part primary size=1000
DiskPart succeeded in creating the specified partition.
DISKPART> active
DiskPart marked the current partition as active.
DISKPART> format fs=fat quick
100 percent completed
DiskPart successfully formatted the volume.
DISKPART> assign
DiskPart successfully assigned the drive letter or mount point.
This creates a 500MB FAT partition as the only one on the disk. list part is only used to make sure I selected the right disk (and the Size shown was from a previous attempt; yours will say something else closer to the full size).
