From the apt-get manpage:

echo libc6 hold | dpkg --set-selections

will hold back libc6, useful when you want to hold back a package from upgrading if you call:

apt-get upgrade

An even easier way from http://www.debian.org/doc/FAQ/ch-pkg_basics.en.html

With aptitude, you can hold a package using aptitude hold package_name and remove the hold with aptitude unhold package_name

Hmmmm, the aptitude command isn’t working for me. I wonder if I’m missing something. Nope, doesn’t seem like it. I’ve never used aptitude before.