The prefilled tax return is a voluntary and free service for you. You do not have to use it - even if you have already applied for it.
After you have applied for prefill data, you will receive a code (Freischaltcode) by post from the tax authorities to activate data retrieval. You can enter this code in the Taxfix app within 90 days.
If you decide not to use the prefill data after all, you can simply choose not to enter the code in the app. The code will automatically expire after 90 days and the service will remain inactive.
You can also easily cancel the prefill data if you no longer want to use it. To do so, simply open the account menu at the top right of the Taxfix app and select "Prefill data consent" -> "Cancel prefill request":