The WP Security Scan plugin will remove version number.
Removing on Versions before 2.6
I can’t think of any legitimate reason why anyone would be running anything other that the latest version of WordPress, but if you are using an older version of WordPress before 2.6, then you will have to manually remove the WordPress version from the theme that you are using. In the header.php file (found in Appearance >> Editor) you will have to look for the Meta tag below and delete it. If it does not exist then there is nothing to remove, otherwise overwrite the version number with whatever text takes your fancy!
1 <meta name=”generator” content=”WordPress <?php bloginfo(‘version’); ?>” />