Here is a solution that works for the asp.net button object. On the front end, add these attributes to your asp:Button definition:

<asp:Button ... OnClientClick="this.disabled=true;" UseSubmitBehavior="false" />

In the back end, in the click event handler method call, add this code to the end (preferably in a finally block)

myButton.Enabled = true;

I have found this, and it works:

btnSave.Attributes.Add(
    "onclick", 
    "this.disabled = true;" + ClientScript.GetPostBackEventReference(btnSave, null) + ";");

Check this link, the most simplest way and it does not disable the validations.

http://aspsnippets.com/Articles/Disable-Button-before-Page-PostBack-in-ASP.Net.aspx

If you have e.g.

  <form id="form1" runat="server">
      <asp:Button ID="Button1" runat="server" Text="Button1" OnClick="Button1_Clicked" />
  </form>

Then you can use

  <script type = "text/javascript">
  function DisableButton() {
      document.getElementById("<%=Button1.ClientID %>").disabled = true;
  }
  window.onbeforeunload = DisableButton;
  </script>

To disable the button if and after validation has succeeded, just as the page is doing a server postback.