One more suggestion would be to add a single line to their plugin to remove the user (by email address) from the first list.
Something like this, to remove users from list 1:
mysql_query("DELETE FROM lm_users WHERE list = '1' AND email = '".addslashes($aMemberEmail)."';",$listmailDBLink) or die(mysql_error());
The variables $aMemberEmail and $listmailDBLink are an example and would need to be replaced with the variable names used by their plugin.
Regards