In another episode, Endora uses magic to give Darren a photographic memory, indicating that witchcraft can be used to alter human memory. But it's never explained why Samantha doesn't simply use magic to wipe the memory of her being a witch from the blackmailer's mind, making him forget the entire incident.
When Samantha pops in on the Leach's apartment and zaps the chandelier right off the chain, you can clearly see that the coffee table breaks in half before the chandelier actually falls on it.