I'm using the datastax version of cassandra and installed it with the command:

   apt-get install cassandra=1.1.9

and, once you do that, apt-get is good about not upgrading any further at all.

But this morning I wasted several hours with hung software on my development machine unto I spotted that "Ubuntu software updater" had upgraded my cassandra to 1.2.x!  AGAIN! ARGH!

After some research this does the trick.

1. create a file /etc/apt/preferences.d/cassandra

2. in it add the lines:

Package: cassandra
Pin: version 1.1.*
Pin-priority: 1000

3. apt-get update

From now on upgrades should only get the 1.1.x versions (it's now at 1.1.11). You can check this with:

   apt-cache policy cassandra

This works fine for the "user friendly" updater too.