You can use the WebDateChooser.MaxDate Property.
In code (which I think suits your needs):
WebDateChooser1.MaxDate = DateTime.Now;
Or in the source (not dynamic so I just added for reference):
<igsch:WebDateChooser ID="WebDateChooser1" runat="server" MaxDate="2009-06-25">
</igsch:WebDateChooser>
UPDATE: (In response to the first comment from @sona)
Here's some code from my source file that accomplishes the setting of the MaxDate property dynamically.
<script runat="server">
void SetMaxDate(object sender , System.EventArgs e)
{
WebDateChooser1.MaxDate = DateTime.Now;
}
</script>
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<igsch:WebDateChooser ID="WebDateChooser1" runat="server" OnInit="SetMaxDate">
</igsch:WebDateChooser>
</div>
</form>
</body>
I can't attest to this being the best approach since my knowledge of asp.net is very limited. Although, it should work for you...