Just add this style in a content editor web part in the page you want to hide your left navigation. In case you need to hide it in all the pages, add this to your master page.
<style type=”text/css”>
body #s4-leftpanel { display: none; }
.s4-ca { margin-left: 0px; }
</style>
The style applied to body #s4-leftpanel hides the left navigation & the style applied to .s4-ca removes the empty space & moves the other content of the page towards the left side.
