No, it isn't. If you mean rabble, as in disorganized crowd, then maybe... it comes close. Rubble though is just a bunch of broken rocks, and wouldn't qualify as an antonym.
The opposite (antonym) of antonym is synonym. An antonym is a word that means the opposite. A synonym is a word that means the same thing, or nearly the same thing, for a given context.
Some words do not have direct opposites [antonyms]. Antonyms do not usually substitute for phrases. The phrase "you are" has no antonym. A contrasting version of "you are" is "you are not".