jquery 阻止表单提交方法

<form name="message_form" action="?m=mobilecenter&c=index&a=service" method="post" onsubmit="javascript:chkservice(this);return false;">

</form>

function chkservice(From){
 var tags = false;
if(tags){
alert('禁止提交');
return false;
 }
 From.submit();
return true;
}