Error message code on BSP Page

Hi experts,

I have one code like this :

<%

IF co_model->firstname CA '+-=<>()'.

%>

<script language = "Javascript">

alert ("First name is not correct")

</script>

<%ENDIF.%>

It is showing only warning message and allowing to save the data of BSP Page when press Save Button.

In need to convert this alert (Warning) message into Error Message,

Please help, it is urgent,

Thanks in advance,

Study SAP