Here is the example to override events using hook in Liferay 6.1.
Specify below property in your liferay-hook.xml
<portal-properties>portal.properties</portal-properties>
Now create portal.properties inside example-hook/docroot/WEB-INF/src.
Specify events you wants to override in portal.properties file
i.e
servlet.service.events.pre=com.sample.portal.kernel.events.CustomServicePreAction
You are done ..!! create CustomServicePreAction to override above event.
