Javascript - Disabling A Label From Client Side?

I have a webform that has a lot of controls and a submit button. When a user submits clean data, there is an asp.net label control that displays a "form submitted successfully" message. My problem is that after this successful submission, if the same user enters some invalid value for some fields on the same form and clicks on the submit button, the page still displays the success message along with the asp.net error message for that field (By using the asp.net validation control). My question is how do I get rid of the success message in this scenario? (In this case all I want to display is the client side validation error message). I am not that familiar with javascript.

<asp:Button ID="btnAddActivity" Text="Add Activity" runat="server" onclick="btnAddActivity_Click" ValidationGroup="vgSetup" OnClientClick=" return changeLabel();"/>
<asp:Label ID="lblMessage" visible="false" ForeColor="red" runat="server"></asp:Label>
function changeLabel()
{
document.getElementById('<%= lblMessage.ClientID %>').innerHTML = "";
return;
}

Here is the markup for the label and button .I tried calling a javascript function on the OnClick event .This disables the success message, but the validation error messages are displayed after a post back.I want to display the validation error messages without a post back.