You can prevent a drag and drop value from your page and a copy paste text in your textbox.
For example: I have a textbox:
<asp:TextBox ID="txtSeqNumber" runat="server" Width="150">
How can I prevent this? Most of the time, this error is reported by testers. The simplest solution is as follows.
<asp:TextBox ID="txtSeqNumber" runat="server" onDrop="blur();return false;"
onpaste="return false" Width="150">
blur() function, the
txtSeqNumber lost its focus and we are not able to drag and drop the text.