You can delete the song in iTunes and download it again for free if purchased from iTunes. The instructions for this are below.
To delete an item from your library:
Click the item to select it, and press the Delete key.
Do one of the following:
To remove the item only from your iTunes library, click Keep File.
To delete the item from your computer, click Move to Recycle Bin. The item is deleted the next time you empty the Recycle Bin.
To download past purchases from iTunes: