function fncKeyStop(evt)
{
if(!window.event)
{
var keycode = evt.keyCode;
var key = String.fromCharCode(keycode).toLowerCase();
if(evt.ctrlKey && key == "v")
{
evt.preventDefault();
evt.stopPropagation();
}
}
}
<asp:TextBox ID="txtSpec" runat="server" CssClass="EditTextBox" Width="367px" onkeydown="fncKeyStop(event)" onpaste="return false" oncontextmenu = "return false;"></asp:TextBox>