If you ever have a hard drive or USB key that has partitions you can't get rid of in Disk Manager, just use the command line tool diskpart.
Make sure to select your disk / partition / volume / vdisk (if necessary) and use the "clean command"
like Delpart, this is a VERY powerful tool that will wipe your drive clean in a second, so use with extreme caution. Make absolutely certain you have the right thing selected before you say "clean" The selected items will have "*" stars by them, see below.
DISKPART> list volume
Volume ### Ltr Label Fs Type Size Status Info
---------- --- ----------- ----- ---------- ------- --------- --------
Volume 0 H DVD-ROM 0 B No Media
Volume 1 DVD-ROM 0 B No Media
Volume 2 System Rese NTFS Partition 100 MB Healthy System
Volume 3 C NTFS Partition 297 GB Healthy Boot
* Volume 4 Removable 0 B Unusable
Happy Partitioning.
