Disallowing content previously indexed

What happens to indexed content subsequently blocked by robots.txt

[daveozzz]

If you have certain pages which have been indexed, and subsequently you decide to block those pages via robots.txt, will search engines eventually drop those pages from the index? (Specifically Google)

I know I can manually remove URLs with the Google URL removal tool as long as they are blocked by robots.txt or return a 404.
It's stated that the pages returning 404 will be dropped eventually anyway. But I couldn't see what happens over time with pages which are just blocked in robots.txt if you do not manually remove the URLs.

[Quadrille]

I don't think it's reliable or rapid.

You may get better results with on-page instructions.

May also be worth removing that page entirely, and placing the content at a new easily-blockable URL

[daveozzz]

Ah good idea.
I've vhanged the unwanted indexed items to return 404 and I've remake them in a way that'll be blocked by robots.txt.
Hopefully that'll sort it out eventually.
Cheers.