For the BizForm_Insert_Before, are you talking about the global event BizFormItemEvents.Insert.Before?
While you may be able to throw an exception and possibly catch it, i think you'll be better off cloning the Form web part and adding your logic in there.
If you are using the BizForm control's insert before event in the ascx.cs file, then i don't see why you will have trouble performing this. I would just (in a cloned copy of the Form web part) add a <asp:literal> tag and if the form submit is invalid, populate the literal with your message and then abort the insert, should be doable. May want to just add the logic to the Validate instead though, i think you can overwrite that...been a while.