If you can get to a command line, such as ssh,
sudo rm -rf /Volume_1/ffp
ought to do it.
Be *very* careful with that command! If you specify the wrong directory, you could lose data that you really didn't mean to delete. Do
ls /Volume_1/ffp
first, and if the files that it lists are the ones you wanted to get rid of, then go ahead with the rm -rf