remove recent posts and categories voyage theme

It puts in default widgets if none are used. Hide it with css (style.css) on all single pages:

.single-post #sidebar_one {
display: none;
}

or remove this from your single.php:

<?php get_sidebar(); ?>

This is essentially the same as the single-fullwidth.php template from the theme.

btw:
consider creating a child theme instead of editing your theme directly – if you upgrade the theme all your modifications will be lost.