As collection projects are more or less a nice visualization of a filter search, I see it is not possible to ‘lock’ certain projects - so that no further observations can be added.
That is, unless the upload date is employed for this. Would be handy, e.g. for bioblitzes, City Nature Challenges or certain school projects, to have a time window where observations have to be made (= observation date), then an extended period where observations can still be uploaded, if they have the right observation date (= date added), and after that no more addition will be possible.
As this functionality is already there in the ‘Explore’ filter settings, I reckon it should be easy to implement for Collection Projects as well.