I agree with you it would be a nice to have functionality, so i would suggest you to go to our Feedback Portal
and propose your feature. The more votes an item gets - the higher priority it will get in our product backlog.
As a workaround you can make all users only with guest permissions. That way, they won't be able to edit the items when the project is finished.
If you have any other ideas do not hesitate to contact us or post them in out Feedback Portal