How do I cancel the PostBackURL property of a button?

Hi guys,
I'm wondering if anybody knows how to cancel the PostBackURL property when a user clicks a button.  I have some code in the button's Command or Click event that I wanted to run first.  But depending on the condition the results from that code, I want to add code that will cancel the PostBackURL property so the button doesn;t do a postback OR cancel the Client or Command event so it doesn't fire up the PostBackURL property.
I cannot take off the PostBackURL property as I am using crosspage functionality.

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