I am trying to customize my left side-bar in Magento when the frontend shows a certain category. For example I want to remove the Newsletter box and a custom block which was added by the theme developer.
However I am not able to find the correct file to edit in order for being able to customize apropriately.
You should do this with XML, by updating your local.xml file.
<catalog_category_default>
<reference name="left">
<remove name="left.newsletter"/>
</reference>
<catalog_category_default>
Refernce for removing common items in Magento: http://myphpinformation.blogspot.in/2015/05/remove-default-magneto-block-like-news.html