How to, using PHP scripting or WordPress overwrite all of the posts (permamently, so it takes effect on database) with some static text such as "Content removed."?
Do not run this on a live site until you're tested it on a backup or local installation and confirmed valid results, i won't take any resposibility for lost data, should that occur.
To run from PhpMyAdmin
UPDATE wp_posts SET post_content = 'your_content' WHERE post_type = 'post'
To use inside a plugin or your functions file (run once)
global $wpdb; $my_update_query = $wpdb->get_result( "UPDATE $wpdb->posts SET post_content = 'your_content' WHERE post_type = 'post'" );
Another option, if you need further control of the query or the replacement function, is to use more of the WP APIs, like this:
$args = array( 'post_type' => "post", 'numberposts' => -1 // additional arguments ); foreach ( get_posts( $args ) as $thispost ) wp_update_post( array( 'ID' => $thispost->ID, 'post_content' => "your new content" ) );
Bear in mind that this is much more resource-intensive than the SQL queries t31os proposed, so use with caution if you're modifying a large number of posts. See also: How to apply content filter permanently?