The correct and only way is to initially allow to crawl the pages. Set the meta tag name="robots" content="noindex,follow" on the affected pages.

The correct and only way is to initially allow to crawl the pages (remove the pages again from robots.txt). Set the meta tag name="robots" content="noindex,follow" on the affected pages. Google will crawl the pages again and remove them from index after a certain time.

The correct and only way is to initially allow to crawl the pages. Set the meta tag name="robots" content="noindex,follow" on the affected pages. When the pages were removed from index, THEN add the disallow in the robots.txt.

In your actual setting you are telling google only this: "Please do not access/recrawl these pages". How should Google know that you want to deindex these pages?

The correct and only way is to initially allow to crawl the pages. Set the meta tag name="robots" content="noindex,follow" on the affected pages.