Use a filter to block a category

/**
 * You can have all posts in a category set as blocked
 * by using the wpmem_block filter hook and WP's has_category().
 * 
 * @see https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_block/
 * @see https://developer.wordpress.org/reference/functions/has_category/
 */
add_filter( 'wpmem_block', function( $block, $args ) {
    
    // What is the category? (can be single, ID, or array)
    $category = 'my-blocked-category-slug';
    
    if ( has_category( $category, $args['post_id'] ) ) { 
        return true;
    }
    
    return $block;
}, 10, 2);