How to sanitize post title with commas

Depending on how you are outputting the title, the answer is different. But there are two possibilities.

By Using wp_title();

If your theme is using the wp_title(); function in its header.php file, you can use the wp_title filter. However, this function is being deprecated since 4.4.

add_filter( 'wp_title', 'filter_the_title' );
function filter_the_title( $title ) {
    $title = str_replace( ' ', ',' , $title);
    return $title;
}

By Using add_theme_support( 'title-tag' );

Newer themes add this method to enable the title tag support. If this is your case, you can use the pre_get_document_title filter to do so.

add_filter( 'pre_get_document_title', 'filter_the_title' );
function filter_the_title( $title ) {
    $title = str_replace( ' ', ',' , $title);
    return $title;
}