On a Chromebook, how do I selectively delete Search entries from the Omnibox

I was having this same issue (I think). The Shift+Alt+Backspace wasn't doing anything on my Samsung Chromebook. Turns out I had saved the url as a bookmark (I'd bookmarked the page) and Chrome was using the bookmark to suggest autocompleting the url.

Deleted the bookmark and Chrome stopped suggesting it. Hope this helps!