I just want to give my 2 cents worth. I've had some stubborn thumbs.db files that were giving me heartache. I tried all that was mentioned above, and the only real solution was to reboot the server.
That being said, I just had an experience where I tried multiple times to delete one of the files, each time it told me I was unable to delete it because it was being used. Low and behold, on the 4th attempt of trying to delete it, within a 10 second time frame, it magically let me delete it.
I tried this on other stubborn Thumb files that were giving me fits. One file took over 20 attempts to get the file deleted. Very odd. I'm not sure if this has anything to do with our Anti-Virus software (Sentinel 1), but thought I would share.