If I understand your question correctly, you use one of the options -yor --yes, or --assume-yes

From the man page http://manpages.ubuntu.com/manpages/utopic/man8/apt-get.8.html

-y, --yes, --assume-yes
Automatic yes to prompts; assume "yes" as answer to all prompts and
run non-interactively. If an undesirable situation, such as
changing a held package, trying to install a unauthenticated
package or removing an essential package occurs then apt-get will
abort. Configuration Item: APT::Get::Assume-Yes.

I suppose it may depend on what you mean by "unauthorized packages"

I would use extreme caution with any of the force options with apt-get, forcing things is a good way to break apt, or your install, or both.