Kind of, but it is not easily. You can log the IP address of logged in users and prevent login from more than one IP address at a time. Sites like Hulu do this and prevent more than 2-3 people watching at the same time. However if you aren't logged in at the exact same time, then it can be used by multiple users. The issue is that you don't know if it is multiple users using the same account or if it is the same user from multiple locations (home, office, cell).
One way to get around it is to put a limit on the number of searches or page views. Then users wouldn't want to share their login information since it would eat up their access.