Solution for Users Email Address Being Used Falsely

Several users in my Domain received a spam email that had a False, From: address of another one of my domain users. It looks like his address book was cracked by spam that then sent out a false email, with link to click on and displayed a false address of a known domain user. Obviously the solution for the receipiants is to not click on link and block and delete email. What solution do I need to do for the user whose address is being used falsely?