You need to use the right hooks (which are not always the same as the URLs/slugs), and it doesn’t hurt to use a hook that runs later (e.g., admin_init):

add_action( 'admin_init', function () {
    remove_menu_page( 'edit.php?post_type=participants-database' );
    remove_menu_page( 'wpcf7' );
});

You can use the following to debug:

add_action( 'admin_init', function () {
    echo '<pre>' . print_r( $GLOBALS[ 'menu' ], true) . '</pre>';
} );

This gives you array of the menu positions and inside positions the array of the active menu items and you just need to pickup the right hook (key [2]) and under that is your target hook or URI.

Here is one helper for this:

if(!function_exists('remove_admin_page')) {
    function remove_admin_page($needle) {
        if(isset($GLOBALS[ 'menu' ]) && !empty($GLOBALS[ 'menu' ]) && !empty($needle)) {

            $needle = strtolower($needle);
            $needle = trim($needle);

            foreach($GLOBALS[ 'menu' ] as $position => $items) {
                foreach($items as $key => $item) {					
                    if(strtolower($item) == $needle) {
                        remove_menu_page( $items[2] );
                    }
                }
            }
        }
    }
}

Just use it like this:

remove_admin_page( 'Participants Database' );

This helper search by name, slug, uri and remove proper menu page.