From Manage invitations on Hangouts
Block someone or report abuse
When you ignore someone, you also can block and/or report the person
Report the person for abuse on Hangouts. You'll be letting Google know
that there may be an issue with your Hangout with this person, such as
harassing you or sending you spam. If we find that someone is
violating our policies, we will take action. Reporting abuse doesn't
block a person. You have to do that separately.
If you didn't have a Hangout with the one that is impersonation you, ask others to report him/her.
Google have consolidated the accounts used to access their services so a Google account is required to save preferences and content in their services like Hangouts, Google+, Youtube, etc.