Your post right here seems like a good example. You could say something along the lines of "This is a dismissal, not an argument; merely naming a bias isn't enough to convince me. If you provide some specific examples, I'd be happy to listen and respond as best as I can." You can even tack on an "But until then, I'm downvoting this because it seems like it's coming from hostility rather than a desire to find the truth together."

Heck, you could even copy that and have it saved somewhere as a form response to comments like that.