Here's a simple hack to prevent registered users from selecting either dragonfly or default themes.
As I'm introducing a new theme, I want to give my users a choice, but didn't want to support the 2 original themes. Nor did I want to remove default and dragonfly entirely as they are useful to me for debugging issues, etc.
Hack is very straightforward, but finding the place in the code where it belongs is not, so I thought I'd share ... Hack was developed for '184.108.40.206 CVS', but will probably apply to all recent versions.
In modules/Your_Account/functions.php find the line