IP address dilemma

Someone was email harrassing my girlfriend and we were trying to figure out who exactly it is... The only evidence I have is there IP address and their email address they used to do it...which i'm assuming isn't their main address its just something they created for this purpose...any help would be appreciated...thanks

options are as follows:
*. change girlfriend
2. change email address
*. change ISP
4. the best option (change all the above i.e *, 2 and *)

best whay its to change your girlfriend :)

BRAVO tWISTED_bRAIN