By default, apt installs the version of the package with the largest version number. You can use APT preferences (wiki) to override this. APT preferences let you configure priorities for package versions based on the package name, the version number, and the source from which the package is installed.
By default, a package source has a priority of 500. If you reduce the priority to 100, then packages from that source will not be installed automatically unless it is necessary to satisfy a dependency. If you reduce the priority to 1, then packages from that source will never be installed unless you explicitly request that version or that source.
Create a file in
/etc/apt/preferences.d and declare packages whose origin is the Debian mirror you use (and
security.debian.org, if you don't want security updates) as having priority 100 (or 1, depending on how badly you want to avoid downloading packages). For example, if you update from
ftp.iitm.ac.in (i.e. if you have lines beginning with
deb ftp://ftp.iitm.ac.in/ in
Pin: origin "ftp.iitm.ac.in"