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 have uninstalled Zeno and want to remove all data, then you can remove Zeno code by following these steps:
Step 1: Select Edit code on your current theme.
If you haven't set a Landing page as home page in Zeno, you can skip this step.
If you have set a Landing page as home page from Zeno Dashboard, then you can revert the original homepage of the theme, by removing the file index.liquid, rename the file index.zeno.json to index.json
Step 3: Search and remove all files which have zeno in their names.