You can effectively “empty” the Recycle Bin from the command line by permanently deleting the Recycle Bin directory on the drive that contains the system files. (In most cases, this will be the C: drive, but you shouldn’t hardcode that value because it won’t always be true. Instead, use the %systemdrive% environment variable.)
The reason that this tactic works is because each drive has a hidden, protected folder with the name $Recycle.bin, which is where the Recycle Bin actually stores the deleted files and folders. When this directory is deleted, Windows automatically creates a new directory.
So, to remove the directory, use the rd command (remove directory) with the /s parameter, which indicates that all of the files and directories within the specified directory should be removed as well:
rd /s %systemdrive%\$Recycle.bin
Do note that this action will permanently delete all files and folders currently in the Recycle Bin from all user accounts. Additionally, you will (obviously) have to run the command from an elevated command prompt in order to have sufficient privileges to perform this action.