And that's where I say, "Changing the TOU to explicitly allow messageThat's a ludicrous answer.
a) Yahoo Groups already had language limiting the license granted to the service. Mark may not want that exact language, but the TOU has already been changed at least once, so there clearly is a practical means for editing it.
b) A practical mechanism for message deletion already exists. A practical mechanism for finding all messages by this member already exists. All that is required for a bulk feature is to marry those two capabilities. Perhaps using a UI similar to the checkboxes present on the /topics-test page.
c) Practicality is Mark's job.
Granted that (b) doesn't cover quotes of the subject member appearing in other members' messages, nor does it cover subject messages stored in other members' email interfaces. But I never asked for either of those things.