I’ve often had the need to address only a subset of people on a mailing list. For example, the devel mailing list of an open source project can have many independent subprojects, so why not allow Gmail-esque “mailinglistaddress+LABEL@yahoogroups.com” filters, where the LABEL can be any word to indicate subset information. Then, people can specify to the mailing list to either allow only emails with that LABEL word(or a set of terms), or to ignore emails with that term. So all you would need to do is reply to the mail whose label you want to ignore by appending a “SYSTEM:IGNORE” to the subject or something like that.
It would be really cool if someone can come up with a nice way to fit this into mailman or some other MLM as a proof of concept. The ignore emails with this label command is quite feasible and user friendly, IMHO.