For this article I will use Ubuntu 12.04 with Gnome Shell 3.4.1. It should be easy to do the same things on your distribution by following these instructions.
First thing you need to do, is to download and install the xscreensaver package. You can find this on your software manager or enter the following command on a terminal:
sudo apt-get install xscreensaver xscreensaver-gl-extra xscreensaver-data-extra
After that, you will be able to run and configure xscreensaver by the launcher found in the applications menu.
When you launch xscreensaver you are offered a choice to disable gnome-screensaver. This will result in the inability to lock your screen. You won’t need to disable gnome-screensaver actually. You just won’t get the black screen, but you will be able to lock your screen any time.
What you need to do next is to make xscreesaver launch at startup. To do so, enter the following command on a terminal:
This will open a new window from where you can enable and disable the startup of various applications. Press the add button and write what you see on the following screenshot.
You can then press the Add button and enable it. This will launch xscreensaver automatically every time your system starts!