For a variety of reasons, the taxonomies that you won't be using need to stay put.

BUT you can HIDE or set these as UNSELECTABLE so that they are essentially invisible to any users.

Open the Taxonomy Manager under your dashboard.

Click on categories, site placement, classification, or topics to choose your taxonomy. Then hover over the taxonomy you want to hide and click on the edit button that appears.

A sidebar will pop up on the right. At the bottom click on Hidden? And uncheck Selectable if it's checked. Hit Save Changes.