Do you have a WELCOME/Netiquette file? If so and "they" post contrary to
your charter, then do what any responsible list-owner would do: either
communicate directly with said offenders, SET them to either REVIEW or
NOPOST, UNSUBSRIBE them, or finally filter= them.
If OTOH you know that they are rogue, simply boot them.
They will be known by what they do post (if you don't boot them).
/P