How to remove the "For Purchase" section from the Software Center?

The Software Center has a section of For Purchase programs —see screenshot below—,
which is annonying me every time I want to install something.

Is there any way to remove or hide these programs from the Software Center?

Links to answers:

For 13.10 and higher→
For 13.04→
For 12.10→
For 12.04→
For 11.10 and lower→
Please leave a comment or send a mail to jmendeth@gmail.com
if it's not working for you or you're having trouble!