Simplest way is to copy any individual files or folders you need to keep from the external drive temporarily to a folder on your internal drive. Then simply format the external drive to completely wipe out everything and start fresh. Then copy back the files from the internal drive if any.
There are instructions for manually removing only the backup files in a 'Sticky' thread at the top of the forum where you posted, but these are a bit complex and sometimes haven't worked well for particular systems.
Rob