I have found 2 possible solutions for this. Either you close the project, then editing won't be available for evaluators, or you can select "Mark as Done..." by the subject's dropdown options (see attachment) and it prevents evaluators from editing their evaluation. (You can also select "Unmark as Done..." should you need it.) The Edit Evaluation button will be still visible, however, evaluators will get a notification saying they are no longer able to change their submitted responses. Hope this will help:)