you can achieve the solution by the following way -

first your textbox is look like as follows -

<asp:TextBox ID="txtNumber" runat="server"
Width="200">

all you have to do is to add the following javascript
code in the textbox control besides writing many codes-

<asp:TextBox ID="txtNumber" runat="server"
onDrop="blur();return false;" onpaste="return false"
Width="200">