unless you trace it while he is on you will probably not get him but someone else. dhcps change constantly especially once someone logs off the internet.
however, you don't need to go that far. if you know they are being sent via Yahoo, then forward the emails to abuse@yahoo.com along with a message that you expect them to take action.
unfortunately, the person may just open another yahoo or hotmail or some other online email address and continue where they left off.
if this is a wrok related problem get your HR or legal deptarment involved. if it is a personal just add their email address to your junk filter.
if it is serious enough to warrant legal action consider a local investigator (preferabbly one with forensics background).