"Declare an embargo on"? "Declare a moratorium on" (Beryl's example, but it seems possible - "Let's declare a culture-wide moratorium on Honey Boo Boo," for instance)?
Really, this isn't a question about "declare," but a question about nouns that take the preposition "on" and that also can be "declared." The reason "declare news on" doesn't work is because, first, we don't say "tell news on" someone, and, second, because we don't talk about "declaring news."
And do we actually say "declare war on [somebody]" - or do we say "declare war on [a political entity presumably made up of somebodies]"? I wouldn't say "We declared war on Hitler" but "we declared war on Hitler's Germany."