Programatically activate/ deactivate onClientClick from Code Behind?

I have a button on my page that I need to programatically activate/deactive an onClientClick event.  Is it possible to control this from my page behind?    You see, depending up a value for another variable, the button may or maynot need the onClientClick.  So, when that variable is set in my code behind, how do I enable/disable the onClientClick?
Here's the code for my button:
<asp:Button ID="btnContinue" runat="server" Text="Save &amp; Continue" ValidationGroup="Insert" OnClientClick="return confirm('Are you sure you want to Insertthis record?')" />
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