It sounds like you're talking about Selenium here and not WPF. Selenium has its own support forum so really you should go there to get help. They can provide better assistance.
But as a starter the information I see says to call Quit
on the session. If you do that then it is supposed to close all the windows it opened. However searching their issue list here seems to indicate there may be some issues with this unless you're doing things the way they expect. I don't use the driver myself so I cannot confirm.