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