When the user visits a page such as the terms-of-use (TOU) agreement form, there is a checkbox which you must agree to the TOU. If that checkbox is not checked, the 'Yes' button should be grayed-out or disabled, preventing the user from clicking it.
Update: Another similar place this should be implemented is the Account Registration Form, where the TOU is present. The checkbox should be checked before the submit button is available.
This would be a more friendly UX.