I know it's an old post but I figure I will chime in as I just had similar problem and found a solution.
I ran Defraggler on my SSD which had around 150 GBS free space. I set it to 'Optimize' and progress reached 74% and computer froze but on the screen I noticed that I had 19GB free space - which I understood it was going to happen during this process. After hard reboot I still had only 19 GBs of free space and over 100 of DFOptimize_xx.bin files on the root of my C drive. I tried different things I found online including this thread but free space didn't increase. So I started deleting them few at the time after I backed them up to another drive. After delete I rebooted and emptied trash. Repeated process few more times and all seemed to be working fine. So I just deleted all those files, emptied trash and reboot. All is still fine. So in a nutshell I could just delete all those files manually without a problem. When you delete it prompts saying that you need administrative permission to delete. so you confirm and done.
I hope this was helpful.
