MyUnity unfortunately up to now is not able to do this due to the new version of Unity in 12.10 (see this related question).
You can use Unsettings to do this. Here you can find the .deb package and if you want you can also add the ppa.
To disable the "Apps available for download", go to the Dash menu and switch off Apps for download.
(click to install)
Configuration of Display Available Apps to download is directly configurable from the Dash
- Open the Dash
- Click the lens as Application Lens
- Click the down arrow to reveal the Filter Results options
- To switch off the display of applications to download click the Local Apps source
- To turn on the display of applications to download click the Software Centre or All
Configuration of Display Available Apps and its counterpart Display Recent Apps is now available directly via the GUI configuration application MyUnity
The image shows the option turned off - you can click as shown to switch-on the Dash display of available applications.