Disable a button control during postback.
Here is very simple (non-AJAX) way to disable a button control during postback:
<asp:Button runat="server" ID="BtnSubmit"
OnClientClick="this.disabled = true; this.value = 'Submitting...';"
UseSubmitBehavior="false"
OnClick="BtnSubmit_Click"
Text="Submit Me!" />
Found here.