You can cancel the GTM subscription by following steps:

1. Log in to your Shopify store admin.

2. Click on Apps from left navigation,

3. Find the Google Tag Manager app in the list

4. Click on the delete button.

5. You will be asked that why you are deleting the app which is optional.

That's it.

there's only one piece of code that needs to be removed, it is in the theme.liquid file as per below


<!-- Google Tag Manager for Shopify by Niddocks - Set Enhanced Ecommerce Start -->
<script>window.localStorage.setItem("ecommercetrackingtype", "ee");</script>
<!-- Google Tag Manager for Shopify by Niddocks - Set Enhanced Ecommerce End -->