lpr -r -s

[Ian Lindsay <iml04_(_at_)_hampshire_(_dot_)_edu>]

lpd ignores unlink cards in the control file when the filename to be
unlinked contains a slash, which is exactly what lpr emits when given
the -r and -s options together as suggested by the manpage. in freebsd
the -s flag is not needed for this purpose as -r causes the spool file
to be hardlinked instead of copied. i suppose allowing lpd to unlink
arbitrary daemon-writable files specified in untrusted control files
is a security risk, though...