How do I permanently delete an app from my iPhone 6S?

You can delete the app following these steps:

1. Find the app which you want to delete on your home screen
2. Hold down on the app
3. Press Remove (in red writing)
4. Select whether you want to Delete App (Removes the app from your phone, freeing up storage meaning you will need to re-install the app if you want to use it again which seems like the solution you are hoping for or "Remove from Home Screen" which keeps the device on your phone (does not free up storage) but it is hidden in the App Library.

Irrigationman2021 wrote:

How do I permanently delete an app from my iPhone 6S?

By deleting your Apple ID, and migrating to a different Apple ID.

Draconian, yes, and undoubtedly not what you really want to have happen.

Other than that, and as correctly stated above, the purchase will remain in your purchase history, whether hidden or not.

If you have an app subscription, you will want to delete that: How to cancel a subscription from Apple - Apple Support

If your actually referring to removing it from your purchase history, the answer is you cannot do that. Users cannot edit their purchase history. You can hide apps in your purchase history, but not delete them.