non-stable packages infestation

[Kynn Jones <kynnjo@gmail.com>]

I'm trying really, really, really hard to keep my system mostly within
stable, but I must be doing something completely wrong, because at the
moment my system is infested with > 100 packages that are not from
stable, and that I am 100000% certain I did not explicitly install.
These uninvited packages are causing me a lot of headaches.

Is there some way, short of reinstalling my whole system from scratch,
to downgrade all these packages to stable?

I welcome any suggestions,

kj

PS1: I know that, in principle, I can downgrade individual packages by
installing the specific version, but I'm talking about ~140 packages
here, and it would be a nightmare to determine the right version for
each one in order to reinstall it.

PS2: BTW, I assume that these non-stable packages come from
wheezy-backports, since this is the only non-stable source I have in
my sources.list, but when I run apt-cache list on any of them, the
output does not tell me the release of the installed version; for
example (`sudo` is one of the non-stable packages that are currently
in my system, and that I know, for sure, that I did not explicitly
install):

    $ apt-cache policy sudo
    sudo:
      Installed: 1.8.5p2-1+nmu2
      Candidate: 1.8.5p2-1+nmu2
      Version table:
     *** 1.8.5p2-1+nmu2 0
            100 /var/lib/dpkg/status
         1.8.5p2-1+nmu1 0
            500 http://debian.csail.mit.edu/debian/ stable/main amd64 Packages
            500 http://ftp.us.debian.org/debian/ stable/main amd64 Packages