So here is the steps on How to Remove the Sidebar from Post Pages :
1. Login To Your Dashboard.
2.Go To Themes And Take Editor from there.
3. Select the Page Template (Page.php) From the list of files on the Right Side.
4. Find <?php get_sidebar(); ?> from the Coding present there And then delete it.
5. Save it and the thing will be done!
Note: For some template, there can be more than 1 sidebars,
So possibly, you will have to delete 2 lines!
<?php get_sidebar(); ?>
<?php include 'sidebar2.php'; ?>
Sidebar2.php can be any name according to the template. And also, Include function can change to Get function.So Try finding the lines manually rather than Finding it from the Page using Search Of the Browser. The lines usually reside near to the last lines of the Code.