It is possible to get your username changed, but it's going to be very difficult to do so.
In the past, some players have traded in Tournament win codes for a username change, but I doubt that Zanz really has the time to go into the database and do that anymore.
Another idea that's been thrown around is for Zanz to create a one-use promo code that's good for a free name change, so that people can pick a more appropriate username. Still a good amount of work for Zanz to implement, but it could happen farther on in the future.
To answer your second question, there is no way to censor one's username anywhere in-game. I would suggest that you refrain from putting a deck into Arena, because it will be pulled for having an inappropriate username.
