This is probably something you will need to do at some point. You want to validate a page and if it doesn’t fit certain criteria then the form should not be saved until the user has met your criteria.
event.returnValue = false;
Tanguy the CRM tool guy (and CRM MVP) put something very interesting in the comments but I thought I would promote it to this very small blog post
The new method in CRM 2011 is the following one:
preventDefault : Cancels the save operation, but all remaining handlers for the event will still be executed.