That's too bad. Maybe there is nothing you can do, since your nod and "mmhmmm" was not noticed. Maybe you have to mention a girlfriend to maintain symmetry, or something. It is uncomfortable to be misunderstood in this way; it's not necessarily all under your control. For what it's worth, I think it's fine to say directly, "I'm not interested in dating you," and let the chips fall where they may, but I understand that makes some people uncomfortable.