software sources - How to remove a repository?

Please let me know how to remove certain repositories in Ubuntu 11.04. I am getting the following errors while updating.

sudo apt-get update

Err http://ppa.launchpad.net natty/main Sources
  404  Not Found
Err http://ppa.launchpad.net natty/main i386 Packages
  404  Not Found
......
E: Some index files failed to download. They have been ignored, or old ones used instead.

You can easily remove software repositories from the "Software & Updates"

• Open "Software & Updates"

• Select tab "Other Software"

From the "Other Software" tab you can add or remove repositories.