The only possible way I know is by locking the MBR.
DiskPart
List Disk [Find the number of your USB drive by its capacity]
Select Disk [USB drive number]
Attributes Disk Set ReadOnly
That should do it.
Alas, it would also lock your drive for writing.
I'm not sure exactly how far, but I think you can still write to files, since its only the MBR that's locked, not the MFT.
EDIT (Nov 11 at 22:20):
I've made some tests (and some recalling), and the results are somewhere between strange to inconsistent.
I created a VHD with 2 partitions.
When I locked the drive it blocked writing to the volumes as well (I was able to write to the volumes, but it if I unplugged and replugged the disk everything would be like before my writing).
Then I remembered the oddity I came across last time I've played with it.
If I set partition 1 to readonly, it would make partition 2 readonly partially.
Then I found an article somewhere in technet that said setting as readonly only one partition out of two on one drive is possible only on dynamic drives.
I'll do some more testing later, maybe I'll find different behaviors when locking drives on dynamic disks.
SECOND EDIT:
Sorry for taking so long, I was swamped with work.
I've tested this quite thoroughly, and documented the steps and results.
It's a bit long, so you may jump to the conclusion at the end regarding the question's subject, but I had to document the inconsistencies.
Configuration - OS Win2008r2, One VHD with two NTFS partitions - 1(Y:), 2(Z:)
1.
Set volume 2 as read only
Diskpart showed that both volumes are read only (but not the disk)
Volume 2 is locked properly (no option to change volume label, create new folder or paste)
Volume 1 stayed open to writing
Both volumes can't be formatted
I put two big files on volume 1 and changed the volume label, explorer indicated the volume is now full and showed the new label
Dismounted and remounted the VHD
All changes made to volume 1 have been erased (the VHD file size stayed like before writing)
Now both volumes are locked properly (can't write, change label or format)
2.
Set volume 2 as read only
Diskpart showed that both volumes are read only (but not the disk)
Volume 2 is locked properly (no option to create new folder or paste)
Volume 1 stayed open to writing
Both volumes can't be formatted
I put two big files on volume 1 and changed the volume label, explorer indicated the volume is full and showed the new label
I removed the read only flag from volume 2
VHD gets resized to the proper size
Dismounted and remounted the VHD
All changes made to volume 1 stayed
3.
Set disk as read only
Diskpart showed that the disk is read only (but not the volumes)
Both volumes are blocked to writing, can't change volume label.
Formatting is blocked.
Dismounted and remounted the VHD
Diskpart showed that the disk is not read only (volumes didn't change)
Both volumes are writable, and the label can be changed.
4.
Set disk and volume 2 as readonly
Diskpart showed that both disk and volumes are readonly
Same behaviors as before
Conclusion:
In order to block changing the label you must set as readonly the relevant volume.
As the tests above shows, setting one volume as readonly set every other volumes that resides on the same disk, but they only take full effect on the other volumes after disconnecting and reconnecting the disk.