If you have a list member with an invalid address (I ended up with one that had 2 concatenated addresses, yielding 2 @ characters which the default parser apparently does not like), the regular bin/remove_members
script won’t work. You can use this trick though:
cd /path/to/mailman
bin/withlist -l listname
>>> m.removeMember('foo@example.combar@example.com')
>>> m.Save()
>>> ^D
See the Mailman wiki for more details and other things to try if even this step doesn’t work.