I swear this has the opposite effect. In more than one instance, I've told someone I'm ace and it's made them more persistent, like they're thinking 'Oh, I can fix that.' No, I assure you, you can't. Please stop. :(
Pull the reverse card on them and treat their attraction like it's something that needs fixing: "Oh, you're attracted to me? I'm so sorry to hear that! Have you tried therapy?"