Currently we don't have a functionality for closing a finished project. You could only set the release and iteration dates of the project to the finished date. You could also remove all users from this project or make them guests, so they could not update any more details at the project.
There is also a feature request
in our Feedback Portal
about this functionality. You could go and vote for the item.
If you have any other questions or ideas, you can contact us at any time.