[hollerith]

before arriving at the above solution, I found a way to remove an URL from the list of candidates for autocompletion. the reason that did not solve my procrastination problem is that the next time I visited the URL, it was added back to the list of candidates.

If I had found a way to arrange for a URL to stay off the list of candidates (even after I visit the URL) I would have used that because, as you point out, it is a good idea to leave auto-completion on for useful web pages.