Can I exclude certain repositories from apt-get upgrade?
You can comment those repositories in /etc/apt/sources.list by putting # at first of lines. also you can disable them in synaptic > setting > repositories > other software.
more information is available at:
Ubuntu repositories help
you can hold a package using
sudo apt-mark hold package_name
and remove the hold with
sudo apt-mark unhold package_name
and also this link will help:
Pinning