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

Well that or let me pick sources for my lens default! Because reddit just overwhelms it with low quality content

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.

-site:reddit.com works for that.

I think it might also be possible to create a new lens from an existing one while blocking a domain.