It is not possible to hide main categories. However, you are able to hide Wallet's default subcategories. To hide default subcategories, please follow the instructions below:
1. Click on the Settings tab in the menu bar.
2. In the Settings menu that appears, click on Categories.
3. Select the main Category to see the default subcategories.
4. Slide the toggle to the left to hide the subcategory.
Now, you will be able to see only the subcategories that you actually use.