Below is a snippet that you can add to your functions.php file to remove the default Gutenberg block patterns.
function fire_theme_support() {
remove_theme_support('core-block-patterns');
}
add_action('after_setup_theme', 'fire_theme_support');This needs to be added to either an init (external link, opens in a new tab) hook or to after_setup_theme (external link, opens in a new tab), and you will most likely place this inside a function, together with the rest of your theme support.