You could try to hack their router & gain access to their internal network but the chances of success are pretty low. Worth a shot to at least nmap scan it though to see what ports are open.
You can report them to their ISP w/ your real email and provide screenshots, any evidence you can. Wait a day, make an email account on gmail or hotmail, and report the IP again by "forwarding" a generated "Please confirm your sign-in attempt" email that includes the IP - backdate everything to about the same time period of them accessing your account. You want to give the impression that this is a recurring activity with the account / IP address. If you haven't heard anything back indicating the account has been terminated, wait a couple more days and send another "forwarded" email from yet another account. By this time the ISP will already have a red flag on the account from your first emails - the additional reports *should* cause the account to be terminated.