Hi Clare,
Thanks for reaching out!
Once an Articulate course has been published and hosted, it will be up to the hosting environment to control the availability of the course. If you need to restrict access to a content that is hosted on your website after a certain period of time has passed, please reach out to the administrator of the website to discuss possible solutions.
Alternatively, other members of the community have also come up with their own solutions using Javascript which you can check out here. Note that these methods are unsupported.
Hope this helps!