How to delete a font in Windows 7 that is still in use?

The best way to ensure that the font is not in use anymore is to try to remove it in Safe Mode.

You can get there by rebooting, pressing F8 before Windows boots and selecting it with the arrow keys...

Unlocker is a brilliant free little application that can help you determine which application has a lock on a file. It also allows you to release the lock, or if it's the Windows itself, delete it on next reboot.

10 second fix via elvated command prompt: Start > CMD > Run As Administrator:

del /f /s /q /a "C:\Windows\fonts\my-font.ttf"

That will force delete the file and that's it.