How to disable and enable magento's default block

Generrally in magento store some of the default functionality and modules are there which comes by default in some location.Some people needs these default blocks and some don't.That means they need to disable those undesire modules or blocks.In this article I am going to show you how to disable some of default modules and blocks of content, like the callout banners in the right and left columns, or the poll, or newsletter signup.Also here you will be getting two different ways for disabling default modules and block content like:

Magento : How To Remove Back To School Block On Right Side That Comes Up After Checkout

Simple You can do by commenting the some codes in catalog.xml file by :
app\design\frontend\base\default\layout\catalog.xml
if you have installed any theme then according to that theme location :
app\design\frontend\default\yourtheme\layout\catalog.xml