You have to do use the following solution:
Client Side:
<script>
function DisableRightClick(event) {
if (event.button == 2) { //For mouse right click
alert("Sorry, Right Click not allowed!!!");
}
}
</script>
Server Side:
if (!Page.IsPostBack)
{
TextBox1.Attributes.Add("onmousedown", "DisableRightClick(event)");
}