Do you mean that you want to remove the menu completely, right?
You can open the layout file that you're using:
ROOT/templates/purity_iii/tpls/default.php
For example, if you're using default layout, open above file and remove / comment this code:
<nav class="t3-navbar navbar-collapse collapse">
<jdoc:include type="<?php echo $this->getParam('navigation_type', 'megamenu') ?>" name="<?php echo $this->getParam('mm_type', 'mainmenu') ?>" />
</nav>