Can you just check the 'View Source' of the page for what is being rendered for the following line:
<asp:ImageButton ID="btnDelete" runat="server" Width="25" Height="25" ImageUrl="images/delete.jpg" CommandArgument='<%#Eval("UserID")%>' CommandName="Delete1" OnClientClick="javascript:if(confirm('Are you sure you want to Delete the Record ?'))return true; else return false;"></asp:ImageButton>
Try changing the OnClientClick to:
OnClientClick="javascript:if(!confirm('Are you sure you want to Delete the Record ?')){return false;}"