How can I block a package installation on Debian?

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http://serverfault.com – For Debian, I'd like to block packages from installation. For instance, I'd like puppet and superusers to be unable to install ruby1.8, even if a package with ruby1.8 in its dependency list is requested for installation: the puppet run would fail and so too would # apt-get install ruby1.8 Is what I want possible? If so, what do I do? (HowTos)