Question:  How do I configure DasBlog so that it only uses one theme, and does not allow people to change the themes?

Answer one:
If you delete the "themes" macro from your hometemplate.blogtemplate text file folks won't be able to change the theme as there won't be a control to allow them to do it.

Answer two:
If you set your blog configuration to cache the first page, and set the default theme you want in your configuration, the next time IIS starts, the option for changing themes will no longer show!