Seems like they are asking to block a domain for an individual query, or to remove it from their forums lens. It is weird that "-reddit" didn't work for them, seems like that should do it to me
You could create a custom Forum lens and disable the default one in favor of your own. I think a great way to do this would be a "derivative lens" feature, that way you could use all of the sources for the Forum lens, minus the ones you want to remove. I will make feature request for that probably.
I think it might also be possible to create a new lens from an existing one while blocking a domain.