Steps:

1. Navigate to Settings > Security > Users
2. Look for the customer you're looking to delete from the account
3. Under Action, hover on the cog-wheel
4. In the drop-down, select Delete User

Additional Information:

Only superusers and account owners have the ability to delete users. However, if you're a superuser and you cannot delete a standard user please refer to this article:

Why can’t a Superuser Delete Other Users?