Apparently someone is using fake e-mails to badmouth me

It started a long while ago actually, and I usually ignored it, but now he's using multiple e-mails posing as people I know.
It's really annoying. Does anyone know a way how I can get rid of this?

Make a new e-mail address.

Are they emails like dfff124gg32gr@someweirdlink.com?

As far as I know, the e-mails are being disguised as his friends' e-mails, and thus he can't tell if it's actually from them or not.

Yeah.
Only it's pretty obvious from the mails themselves that they're not from my friends, but from the same guy.

Open the headers on the email, find the IP address, and report him to his/her ISP.