This will popup a message when someone tries to PM a member of staff.

<script>
function pm_user(member, message){
    if(location.href.match(/\/pm_send\//) && location.href.match(member)){
        alert(message);
    }
}

pm_user('admin', 'Please Do Not PM the Admin!');
pm_user('god','I really am not God!');
</script>

Main Header

This line: pm_user('god','I really am not God!');
should be repeated.
replace 'god' with the name of the member/staff member and then the other part: 'I really am not God!' with the message you want to display.