Hi,
Try to this java script code:
<script type="text/javascript">
function alpha(e)
{
var k;
document.all ? k = e.keyCode : k = e.which;
return ((k > 64 && k < 91) || (k > 96 && k < 123) || k == 8 || k==32 || k==13);
}
</script>
<asp:textbox id="txtpartyhallname" runat="server" width="200px" cssclass="WaterMarkedTextBox" onkeydown="return alpha(event)" xmlns:asp="#unknown"></asp:textbox>