Obviously this means we'd have to maintain a list of such sites. Yes, I'm aware of what's called the genetic fallacy, but things have changed since rhetoric was invented. We now must deal with dedicated professional creators of lies, and treating their spew as though it were reasonable takes us more time than it does to create a lie.
I claim that it is reasonable to filter by source once we can categorically identify the source as unreliable.
Edit: "no notable claim" is used in several cases, including "we can't tell what you're asking", and "this isn't a question". The new close flag "Source is unreliable" (however it's spelled) is much more emphatic.