return false on page change

Hello,
I managed to trigger a function when a user changes a page (ie: tab) in a dialog of a plugin that I wrote, here it is:

onLoad: function() {
   theDialog = this;

   // Act on tab switching
   theDialog.on('selectPage', function (object) {
      alert('page changed!');
      return false;
   });
}

It works and I can see the alert, but the [return false;] part, doesn't work.

The tabs in my plugin are dependent on each other and I need to prevent the tab change if user forgets to fill out info in one tab, before moving on to the next tab.

Do you have any idea how to prevent the change of the tab?
Thanks!