If you are BigCommerce user, then Zeno will not populate any code into your store theme. So nothing to do!
For Shopify users, Zeno does insert some code into the theme for its functionality.
Unless you have uninstalled Zeno from your store, you should not remove Zeno code.
If you did, then you can remove Zeno code by these step:
Step 1: Select Edit code on your current theme.
Step 2: If you have set a regular page as homepage from Zeno Dashboard, then you can revert original homepage by selecting original version of file index.liquid
Step 3: If you want to keep all pages created with Zeno, you can skip here. If not, then you can remove all of them by finding and remove all files ending with .zeno.liquid or beginning with zeno-