Unfortunately you cannot. Most imposters are banned, but if you send them a link to a random thread on Sythe, and see if they show up on the "Currently Active Users Viewing This Thread", then report that account. But as for imposters with no Sythe account, best way is to report that email/name to the person being impostered.
If you find someone has an impostor, it would be best to notify the user that they are pretending to be so he or she can issue a warning.
