Podrías colocar un código algo asi:
Cita: <script language="javascript" type="text/javascript">
var message="Mensaje a mostrar";function clickIE4(){if(event.button==2){alert(message);retu rn false;}}
function clickNS4(e){if(document.layers||document.getElemen tById&&!document.all){if(e.which==2||e.which==3){a lert(message);return false;}}}
if(document.layers){document.captureEvents(Event.M OUSEDOWN);document.onmousedown=clickNS4;}
else if(document.all&&!document.getElementById){documen t.onmousedown=clickIE4;}
document.oncontextmenu=new Function("alert(message);return false")
</script>
y evitar el clic derecho.