How can I clean out an already existing course to reuse it?

1. Use the Control Panel > Recycle command which is specifically designed to let you clean out certain parts of a course that change from term to term, e.g. student accounts, discussion board entries, etc. This is a very useful command.
2. Note that it cleans out the course as it is, so the data you are cleaning out is gone forever. It is a very, very good idea to first Archive a course before Recycling it.