Obviously Page.ClientScript.GetPostBackEventReference won't work because its on the server, after the button has been clicked and the postback has occcured.
<asp:Button id='btnVerifyEmail' runat='server' Text='Verify Email' onclientclick='return CheckForRequiredFields(this)'/>
function CheckForRequiredFields(btn)
{
btn.disabled = isValid();
return !btn.disabled;
}
Whatever you validation function is if it returns true it will set the disabled property to true and return false which will not allow the postback to occur.