I've removed various posts from my site and changed them from published to draft. Unfortunately they still exist when going from post to draft, 'cos the Search Engines find them at the original url.
Suggestion: I'm not sure how posts that are originally draft stay draft - just created a test draft post and copied and pasted its url and I get a 404 so that works fine - but couldn't the same thing be applied to posts that WERE published and are now draft?
Just done a quick test - created a test post, made it draft, copied and pasted url, get 404 error.
Changed post to published, coped and pasted url, post shows up fine.
Changed post to draft, copied and pasted url, post STILL shows up fine. Oops!
Seems for the time being the only way out is to password protect posts that WERE published and are now draft.
Incidentally I keep them as draft so that I don't lose the information, links etc. in case I want to rehash them and publish later.
This also has implications for people who accidentally press 'Publish' and then immediately realise their mistake and change it to draft - unknown to them their post is now visible on the internet.