For forbidding updating it, you could pin the package.
First find out what priority it's being installed in with. here's an example
$ sudo apt-cache policy skype
*** 184.108.40.206-1~precise 0
500 http://ppa.launchpad.net/upubuntu-com/chat/ubuntu/ precise/main amd64 Packages
500 http://archive.canonical.com/ubuntu/ precise/partner amd64 Packages
Then you can create a file. The lowest priority in this example is 100, so anything below that should make the package not upgrade, despite newer versions being available.
Ubuntu looks for pinning instructions in /etc/apt/preferences.d/. For example, if you are using Ubuntu Precise (12.04), you can create this file telling it to set all packages named skype for precise with priority 90, which will make it prefer the current installed package (which has priority at least 100).
$ cat /etc/apt/preferences.d/skype
Pin: release n=precise
I was told you can do this with synaptic as well.
As for downloading the package, once you've pinned the version you want:
$ apt-get download skype