want to undo apt-pinning

[Benjamin Rutt <brutt+news@bloomington.in.us>]

I had a debian stable system that was running well.  Then, I wanted to
try out some parts of the unstable packages, but without fully going
to unstable.  So I followed instructions for apt-pinning at

http://jaqque.sbih.org/kplug/apt-pinning.html

and installed some packages via e.g. "apt-get -t unstable install
foo".

But now, the C library has been replaced, and I'm having problems
starting some applications, and I have decided that I want to
downgrade back to stable.  How can I clean all unstable packages from
my system, and go back to stable?  Is there way dpkg can tell whether
a package came from {stable, testing, unstable}?  Or can I do that
somehow with apt?

Thanks,
-- 
Benjamin