The handler receives the page number that is about to be activated and should return true to indicate that the activate operation was completely handled and the original page change is not to occur. Return false or null to indicate for normal page activation logic to continue to fire.
A client handler should look like this.
function ClientActivate(num) { if (num == 3) return false; else ActivateTab(3); return true; }
o.wwWebTabControl.OnClientActivate