Correct me if I'm wrong but wouldn't the analogy go more like this: your client (Knots, Core pre v30) filters the spam but Google (e.g. MARA) decides to put it back to your inbox anyway with no way for you to delete it? I don't want illegal stuff on my node and I do not have a good solution to propose but I think it's more complicated than just client side filtering.
Discussion
This is a pretty close. I would say email filtering does not really apply here, it's too different.
You have an old paper archive and stop all customers that do not come with paper but every 10 days a huge dumptruck arrives and unloads on the property.