Can't delete font files
This Registry key manages fonts that the system knows about:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts
I discovered in an older answer that removing a value of that key or changing a value's data to point to a nonexistent file will make Windows not see the font as usable.
Therefore, if you remove the Registry entries that correspond to the fonts you want to torch (and then restart to make the system reload everything), you should be able to delete the fonts' files.
If the above does not work on Windows 10, you can also STOP and DISABLE the following two services:
- Windows Font Cache Service
- Windows Presentation Foundation Font Cache 3.0.0.0
Reboot your computer, delete the font files, and re-enable the services by setting the Startup Type back to 'Manual'.