Note that this filter function, added to the theme functions.php file, will apply to all output that uses “the_content” everywhere (all posts essentially) – so exercise or consider narrowing the scope.
For instance, you might want to limit it to the first 100 characters, or to posts before a certain date, or within a certain category before a certain date, etc., otherwise someday someone may happen to use the same sequence of characters and be shocked to find that everything up to them has apparently been deleted.
/**
* Filter all posts to remove 'Post - '
* and all characters preceding it
* from all posts
**/
add_filter( 'the_content', 'get_rid_of_text_to_post_dash' );
function get_rid_of_text_to_post_dash( $content ) {
$key_string = 'Post - ';
$new_content = ( strpos( $content, $key_string ) ) ?
substr( $content, strpos( $content, $key_string) + 7 ) :
$content;
return $new_content;
}