To be able to delete users, you need the Administrator role (or an individual role with appropriate permissions).
Open the desktop or web client.
Click the small gear icon at the bottom left.
Then select Organization settings.
Click on the Users tab.
Move the mouse over the user you want to delete. Using the Remove from organization option (the trash can as a pictogram), you can now remove the user.
After the user is removed from the organization, he is permanently deleted. It is not possible to restore the user afterwards.