Gnus: expiring unread articles

[Mark T.B. Carroll]

Hopefully, a simpler question than the last:

I have a group called nnml+mail:mail.spam that my probable spam is
filtered into. I am happy to let articles in it expire without my having
marked them read or suchlike.

How do I achieve this? Can I get the articles marked as expirable
somehow when they are filtered into the group? Or can I use some
combination of options to let unread articles expire for that group?

Thanks.

-- Mark