Disable default event observers in Magento

Simply set the "type" to "disabled". For example, if there's a default observer set like this:

<?xml version="1.0"?>
<config>
    <modules>
        <Mage_Reports>
            <version>0.7.10</version>
        </Mage_Reports>
    </modules>
    <frontend>
        <events>
            <wishlist_add_product>
                <observers>
                    <reports>
                        <class>reports/event_observer</class>
                        <method>wishlistAddProduct</method>
                    </reports>
                </observers>
            </wishlist_add_product>
        </events>
    </frontend>

</config>

Then you'd use something like this in your extension's config.xml:

<?xml version="1.0"?>
<config>
    <modules>
        <MyNamespace_MyModule>
            <version>0.1.0</version>
        </MyNamespace_MyModule>
    </modules>
    <frontend> <!-- Be careful to use <frontend> like above, <global> would not work -->
        <events>
            <wishlist_add_product>
                <observers>
                    <reports>
                        <type>disabled</type>
                    </reports>
                </observers>
            </wishlist_add_product>
        </events>
    </frontend>
</config>

Make sure the scope matches though!

Source: http://www.bubblecode.net/en/2012/01/21/disable-an-event-observer-defined-by-default-in-magento/