To be fair, that Varric alone saying "no" should be enough. He's not interested by Cass, he's still into Bianca, and that's pretty much all there is to say on the matter for me.
Oh, but it's not enough. Not nearly.
Someone flat-out saying "no, it won't happen" and having known feelings for a different character aren't enough at all, because there are always going to be people who want to peddle their nonsense and claim it as valid truth.
Just don't say anything to them about it because then they'll get upset and defensive like the barely-out-of-puberty teenage girls they are. I mean, you're not allowed to criticize something they like, it's just their opinion. It doesn't matter if it's outright contradicted in the actual game itself, it's fiction, so any notion of realistic continuity and character interaction go right the f**k out the window, becuase it's not real.
They're free to believe whatever they want, by all means, but to say to someone's face (or to their computer screen, or whatever) that it has some merit of truth is false.