If you want to remove the entire left and right sidebar that is in the default ZenCart table layout, you can do this by setting the following code flags

$flag_disable_left = true;
$flag_disable_right = true;

to the top of the file ...

includes/templates/template_default/common/tpl_main_page.php

or in the corresponding file in your template override folder.

Similarly if you want to remove the header or footer, you can add the following code flags ...

$flag_disable_header = true;
$flag_disable_footer = true;