Your design package directory, in
app/design/frontend/<your_package>/<your_theme>/layout
Create a file named local.xml.
Inside of local.xml, create a <default> block that will contain and consolidate
your global changes:
<?xml version="1.0" ?>
<layout>
<default>
<!-- Your block overrides will go here -->
</default>
</layout>
Depending on what you want to turn off, local.xml might contain some of the following lines.
<default>
<remove name="left.permanent.callout" /> <!--the dog-->
<remove name="right.permanent.callout" /> <!--back to school-->
<remove name="catalog.compare.sidebar" /> <!--product compare-->
<remove name="paypal.partner.right.logo" /> <!--paypal logo-->
<remove name="cart_sidebar" /> <!--cart sidebar-->
<remove name="left.reports.product.viewed" /> <!--recently viewed prod-->
<remove name="right.reports.product.viewed" /> <!--recently viewed prod-->
<remove name="right.reports.product.compared" /> <!--
recently compared prod-->
</default>