For the style account/system that you have, the store name is always part of the URL. We may be able to change the username, but I cannot provide an ETA at this time on doing so as we generally cannot tackle these special projects when there are a lot of eBay changes in the pipeline to adapt to (which unfortunately there are a lot of currently.)
If you could send in an official request from the Help section in your account, put it Attention to me and ill get the request logged for you.
