My "nitpick" was on the use of the word "server". The domain name used for a username is decoupled from the server. But no, there is nothing stopping you from making a stephenking subdomain on another domain. Just like there is nothing stopping you from making a website at google.mycoolwebsite.xyz.

But there are moderation lists that you as a user can subscribe to. It wouldn't be hard to find a moderation list for impersonators, which would solve this problem for you.