Tracing IPs.

can anyone tell me if there’s a plugin or a hack or something, so that I can see if a user is giving out there username and pass to someone else that’s not on the same IP. what i’m trying to say is I’d like to cut down on account piggybacking. I just found out that someone is using someone elses account to gain access to my site, I don’t like that or want that, I’d like to find who the user is and IP so I can deal with them directly?