Hi Jane, you can't prevent it, but you can try to make it more difficult. For example two-factor authentication could be useful. The person has to provide both a username and password AND a second factor, such as a code generated from an authentication app or via SMS.
Lambda actually has a blog post about this here: http://blog.lambdasolutions.net/prevent-cheating-moodle-two-step-authentication
The Wikipedia article on this is also useful to help understand the concept: http://en.wikipedia.org/wiki/Two_factor_authentication