How do I completely turnoff the voice recording feature in the iMessage Reply?

Depends on what exactly you are referring to.

If you mean the dictation option microphone in the keyboard, you can turn that off from Settings ➜ General ➜ Keyboard ➜ Enable Dictation.

If you mean the icon in the reply box to record an audio message rather than dictate one that turns into text, then it cannot be disabled. Simply ignore it, and start typing a response and it will go away while you type and until you send the message.

You can stop it from automatically recording an audio message when you bring the phone up to your ear, by going into Settings ➜ Messages ➜ Raise to Listen and turning it off.

You can’t. Just don’t touch the mic icon.