You can now forbid that employees can make time entries for customers without choosing a project. You find the setting in the General settings.
When you check the box „Track for customers“ then the customers will appear without a project again when you make a new entry. Do not check the box, and the option for a time entry for a customer without a project will disappear from the dropdown menu. Employees will then only be able to choose a customer combined with a project for a new time entry.