I made a greasemonkey script for mm2 that would mark all held messages for discard and also add the message senders to the discard list (https://github.com/bd808/userscripts/blob/gh-pages/mailman-discard-and-ban.user.js). This made dealing with list spammers a relatively easy action on the low volume lists I admin.
In the mm3 UI, the actions to add a sender to the discard list are not directly on the /held_messages screen. Instead you have to click into each sender to add them to the discard list. This is fine, if there are only 1-2 held messages to deal with, but less fine when there are lots of them (spammers come in waves in my experience).
It would be very nice to be able to mark senders for discard from the /held_messages screen directly at the same time messages are discarded.