Two ways:
 
SET listname DIGEST FOR userid@node
 
assuming you support the DIGEST feature, or
 
DEL listname userid@node
 
It probably would not do you any good to pleade with their
postmaster to stop generating such useless messages to
list-owners who have better things to do rather processing
messages that they have no control over, that no one cares
about, and that generates useless network traffic.
 
Pete Weiss