I can no longer use my Kingston DTSE9 flash drive because it is 'write-protected'. How can I remove the write-protection? Disk Utility won't allow me to Erase or Repair the drive, it will only let me verify it. 

Clicking on Disk Partition, I get the following message:
This partition cannot be modified. This disk is not writable and can not be partitioned.

I used this thumb drive to transfer files from my mac to a PC, and it became write protected after a transfer from one to the other.
Before you do anything serious...

Rarely some memory cards/Pen Drives are having physical button/switch (yes a miniature mechanical switch) to make it write protect.. 

Check your pendrive to make sure you did not accidentally activate it by puling/pushing the USB.