I'd like to suggest an improvement. See this article: https://moodle.org/mod/forum/discuss.php?d=244591#p1064680
Basically I want to lock the course once the user has completed it. You can't do this any other way - I can't have it date-dependant. It's all explained in that thread anyway. I suppose that actually, thinking about it, I'd like to be able to lock it on completion, and have the opportunity to have it as read-only, or in review mode, or to just completely lock it.