If you're logged in as root and still cannot delete the directory, assuming the filesystem is mounted rw and securelevel is set to -1, you might have some filesystem corruption that is prohibiting you from deleting the directory. I haven't seen this on a FreeBSD system before however I've had to deal with this many times on Solaris systems. In those cases the system ran nicely however at reboot the filesystem in question was so badly corrupted it required a newfs and restore from backup.