I assume that most people who read this are afraid to answer with a simple: There is not way, it cannot be done. Because there will be some way for sure, but it is probably complicated enough that comes close to a "it cannot be done".
Basically, an email sender can place anything she wants into the from field. At least the visible part. It is also possible to send emails with forged senders in the invisible part, because when email was invented, the inventors did not take all the criminals into account who are prone to abuse the Internet for their nasty stuff by the millions each day.
If you are receiving the mails from a local account (one that is located on the same server), I suggest to ask the user to stop or to leave. If you are receiving such mails from an external source, I recommend to use SPF lookup in the general mail settings to block all incoming mails that have no SPF record in their domains or an SPF record that does not match the sender's IP address. This will block most of such forged mails.
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