I had only replied to the OP because I didn't think other people could
be interested, and I had announced it a while ago already, but here goes
again, just in case.

I patched p9p acme Mail to have a sort by conversation/subject
(http://codereview.appspot.com/264043/show), so if what you want is being
able to delete a bunch of messages with the same subject it's very easy
to do in one mouse swipe once they're all grouped together by this sort.

Of course that does not really help with spam because they won't have
the same subject, only similar ones all matching "viagra", as in your
example.

I've been using it the whole time since then, no issue so far.

Hope that helps,
Mathieu