Hi Gary, the domain name is added before the username by default and this cannot be changed. Kentico uses this to distinguish between and know whether the domain user exists or not. So to answer your question whether you can run SQL script to trim the domain from username, the answer is no.
When displaying the usernames on the live site however, or anywhere else, you can trim and modify them there. It however depends what exactly you're doing with them and how they are being displayed. Let me know what the use case is here and I can give you a more tailored answer.