<asp:TextBox ID="TextBox3" runat="server" onpaste="return false" oncut="return false"></asp:TextBox>
<asp:TextBox ID="TextBox4" runat="server" onpaste="return false" oncut="return false" oncopy="return false" ></asp:TextBox>
forums.asp.net/t/1124202.aspx
forums.asp.net/t/1125227.aspx/1