Difficulty downgrading a mistakenly installed non-stable package

[Kynn Jones <kynnjo@gmail.com>]

I try to keep my system as close to 100% stable as possible.  In spite
of this, a version of libp11-kit0 that is ahead of stable "somehow"
snuck into my system:

    $ apt-cache policy libp11-kit0
    libp11-kit0:
      Installed: 0.20.7-1~bpo70+1
      Candidate: 0.20.7-1~bpo70+1
      Version table:
     *** 0.20.7-1~bpo70+1 0
            100 /var/lib/dpkg/status
         0.12-3 0
            500 http://debian.csail.mit.edu/debian/ stable/main amd64 Packages
            500 http://ftp.us.debian.org/debian/ stable/main amd64 Packages


I tried unsuccessfully to downgrade libp11-kit0 to the version from
stable, like this:

    $ aptitude reinstall -f libp11-kit0
    The following packages will be REINSTALLED:
      libp11-kit0
    0 packages upgraded, 0 newly installed, 1 reinstalled, 0 to remove
and 0 not upgraded.
    Need to get 0 B of archives. After unpacking 0 B will be used.
    E: I wasn't able to locate a file for the libp11-kit0 package.
This might mean you need to manually fix this package.
    E: I wasn't able to locate a file for the libp11-kit0 package.
This might mean you need to manually fix this package.
    E: Internal error: couldn't generate list of packages to download

I didn't have more luck with dpkg:

    $ dpkg --force-remove-reinstreq --purge libp11-kit0
    dpkg: dependency problems prevent removal of libp11-kit0:amd64:
     gnome-shell depends on libp11-kit0 (>= 0.2); however:
      Package libp11-kit0:amd64 is to be removed.
     libgnutls26:amd64 depends on libp11-kit0 (>= 0.11).
     libgck-1-0 depends on libp11-kit0 (>= 0.11).
     nautilus-sendto-empathy depends on libp11-kit0 (>= 0.2).
     libgnutls-openssl27:amd64 depends on libp11-kit0 (>= 0.11).
     glib-networking:amd64 depends on libp11-kit0 (>= 0.11).
     libgnutls-deb0-28:amd64 depends on libp11-kit0 (>= 0.20.7).
     libgcr-3-1 depends on libp11-kit0 (>= 0.6).

    dpkg: error processing libp11-kit0:amd64 (--purge):
     dependency problems - not removing
    Errors were encountered while processing:
     libp11-kit0:amd64

How can I downgrade this libp11-kit0?

Thanks in advance!

kj