If the computer has multiple user account; to clear the recycle bin using command line, you need to get first the SID account of the user.
And the user account that is used to clear the recycle bin should have the proper privileges to access the recycle bin folder.
In Windows 7 and even in Windows 8, to get the user SID is quite straight forward using the command line.
Open a command prompt or press "Windows key" + "R" key to open RUN box and type "cmd" without quotes and press enter or click ok.
It will open the command prompt and type: whoami /user
From the output the user name and the SID will be displayed, copy the SID to clipboard or to notepad.
To copy from the command prompt, position the cursor from the first character where you want to start copying then right click, the select “Mark” a white box will appear drag over to the last character you want to copy and just press enter.
After pressing enter, the mark lines or selected line will be copied to clipboard.
If the default setting of Windows 7 has not yet been changed, type cd c: to go to the c prompt.
On the root of c drive, type this: cd $RECYCLE.BIN
Command prompt will change to c:\$RECYCLE.BIN>
On the recycle bin folder, type: cd user_sid_account
User SID account will be something like this: S-1-5-21-6110774557-2345111707-7778882534-4899
Type DIR to see what's inside the recycle bin folder.
To delete or empty the recycle bin, type Del *
It will prompt whether you want to confirm the deletion or not, type Y if you want to delete N to cancel.
If some files remains after deleting once the system is rebooted it will be gone.
That's it if you want to clear recycle bin using command line.