All documents in Word must have styles. You can't have a document without them, nor a template without them.
Spend as much time as you can learning how to use them to your advantage in your template. They are a powerful tool.
Understanding Styles in Microsoft Word

That said, you can certainly control which styles show up in the quick styles gallery and in various style lists.
How to control the Quick Styles gallery on the Home tab in Word by Shauna Kelly
I would focus on this rather than on deleting styles. Many of them cannot be deleted, anyway.

You can also attach keyboard shortcuts to your custom styles and save those shortcuts in your template. (The shortcuts will work in documents created based on the template so long as the attachment is maintained.)
What content stays in a document created from a template when the template is no longer attached to the document?

You may want to modify the built-in heading styles. You won't be able to delete them, anyway. They can truly seem like magic in a Word document when properly used.
Why Use Word's Built-In Heading Styles? by Shauna Kelly