Firstly, see this page. Method 4 may be able to correct the problem
If there isn't a physical switch to make the USB read only this could be a firmware fault but I don't understand why diskpart said the drive was not read only.
If a firmware fault there is no way to make the drive read-write outside of burning new firmware.
Booting a live linux partition manager called Gparted may be able to let you delete the partitions and reformat the drive. Burn the iso to a CD. Boot Gparted. In the upper right is a dropdown box where you select the device. If you only have one hard drive in the computer it would be sdb, sda being the hard drive. You need to make sure you are dealing with the correct device.
Delete all partitions by creating a new partition table then create a primary NTFS partition. You need to click the green checkmark or Apply to finish all the steps.
For the price of a USB flash drive it may not be worth it though.
Select Device: Make sure it is the correct device
Create New Partition Table
Create a Primary Partition formatted NTFS. Partition > New
If you can't do the above with Gparted then I would say your USB flash drive is finished.