Two options:

1. Keep the demo products installed, and install another copy of Zen Cart for your real activities. That way you can refer to demo products and how they're configured when trying to set up your own items.

2. Re-run the zc_install process and click "Install" to re-install the database contents ... wiping out everything already set in the admin etc. This gives you a clean slate. On the Store-Setup page, tell it to NOT install the demo products.

3. You could simply delete the categories/products and all the attribute option name/values and sale/special/featured tax settings from the admin and start where you left off. This doesn't reset any counters to 0 though.

Hmmmm ... I guess that's three options for the price of two !!